PostgreSQL TODO List

Current maintainer: Bruce Momjian (bruce@momjian.us)
Last updated: Thu Mar 6 17:04:50 EST 2008

The most recent version of this document can be viewed at
http://www.postgresql.org/docs/faqs.TODO.html.

A hyphen, "-", marks changes that will appear in the upcoming 8.3 release.
A percent sign, "%", marks items that are easier to implement.

Bracketed items, "[]", have more detail.

This list contains all known PostgreSQL bugs and feature requests. If
you would like to work on an item, please read the Developer's FAQ
first. There is also a developer's wiki at
http://developer.postgresql.org.

Administration

• Allow administrators to safely terminate individual sessions either via an SQL function or SIGTERM

  Lock table corruption following SIGTERM of an individual backend has been reported in 8.0. A possible cause was fixed in 8.1, but it is unknown whether other problems exist. This item mostly requires additional testing rather than of writing any new code.

  http://archives.postgresql.org/pgsql-hackers/2006-08/msg00174.php

• Check for unreferenced table files created by transactions that were in-progress when the server terminated abruptly

  http://archives.postgresql.org/pgsql-patches/2006-06/msg00096.php

• Set proper permissions on non-system schemas during db creation

  Currently all schemas are owned by the super-user because they are copied from the template1 database. However, since all objects are inherited from the template database, it is not clear that setting schemas to the db owner is correct.

• Add function to report the time of the most recent server reload
• Allow statistics collector information to be pulled from the collector process directly, rather than requiring the collector to write a filesystem file twice a second?
• Allow log_min_messages to be specified on a per-module basis

  This would allow administrators to see more detailed information from specific sections of the backend, e.g. checkpoints, autovacuum, etc. Another idea is to allow separate configuration files for each module, or allow arbitrary SET commands to be passed to them.

• Simplify ability to create partitioned tables

  This would allow creation of partitioned tables without requiring creation of triggers or rules for INSERT/UPDATE/DELETE, and constraints for rapid partition selection. Options could include range and hash partition selection.

  http://archives.postgresql.org/pgsql-hackers/2007-03/msg00375.php http://archives.postgresql.org/pgsql-hackers/2007-04/msg00151.php

• Allow auto-selection of partitioned tables for min/max() operations
• Allow more complex user/database default GUC settings

  Currently ALTER USER and ALTER DATABASE support per-user and per-database defaults. Consider adding per-user-and-database defaults so things like search_path can be defaulted for a specific user connecting to a specific database.

• Allow custom variable classes that can restrict who can set the values

  http://archives.postgresql.org/pgsql-hackers/2006-11/msg00911.php

• Implement the SQL standard mechanism whereby REVOKE ROLE revokes only the privilege granted by the invoking role, and not those granted by other roles

  http://archives.postgresql.org/pgsql-bugs/2007-05/msg00010.php

• Allow SSL authentication/encryption over unix domain sockets

  http://archives.postgresql.org/pgsql-hackers/2007-12/msg00924.php

• Allow client certificate names to be checked against the client hostname

  This is already implemented in libpq/fe-secure.c::verify_peer_name_matches_certificate() but the code is commented out.

• Configuration files
  • Allow pg_hba.conf to specify host names along with IP addresses

    Host name lookup could occur when the postmaster reads the pg_hba.conf file, or when the backend starts. Another solution would be to reverse lookup the connection IP and check that hostname against the host names in pg_hba.conf. We could also then check that the host name maps to the IP address.

  • %Allow postgresql.conf file values to be changed via an SQL API, perhaps using SET GLOBAL
  • Allow the server to be stopped/restarted via an SQL API
  • Issue a warning if a change-on-restart-only postgresql.conf value is modified and the server config files are reloaded
• Tablespaces
  • Allow a database in tablespace t1 with tables created in tablespace t2 to be used as a template for a new database created with default tablespace t2

    Currently all objects in the default database tablespace must have default tablespace specifications. This is because new databases are created by copying directories. If you mix default tablespace tables and tablespace-specified tables in the same directory, creating a new database from such a mixed directory would create a new database with tables that had incorrect explicit tablespaces. To fix this would require modifying pg_class in the newly copied database, which we don't currently do.

  • Allow reporting of which objects are in which tablespaces

    This item is difficult because a tablespace can contain objects from multiple databases. There is a server-side function that returns the databases which use a specific tablespace, so this requires a tool that will call that function and connect to each database to find the objects in each database for that tablespace.

  • Allow WAL replay of CREATE TABLESPACE to work when the directory structure on the recovery computer is different from the original
  • Allow per-tablespace quotas
• Point-In-Time Recovery (PITR)
  • Allow a warm standby system to also allow read-only statements [pitr]

    http://archives.postgresql.org/pgsql-hackers/2007-03/msg00050.php

  • %Create dump tool for write-ahead logs for use in determining transaction id for point-in-time recovery

    This is useful for checking PITR recovery.

  • Allow recovery.conf to support the same syntax as postgresql.conf, including quoting

    http://archives.postgresql.org/pgsql-hackers/2006-12/msg00497.php

Data Types

• Change NUMERIC to enforce the maximum precision
• Reduce storage space for small NUMERICs

  http://archives.postgresql.org/pgsql-hackers/2007-02/msg01331.php http://archives.postgresql.org/pgsql-patches/2007-02/msg00505.php

• Fix data types where equality comparison isn't intuitive, e.g. box
• Add support for public SYNONYMs

  http://archives.postgresql.org/pgsql-hackers/2006-03/msg00519.php

• Fix CREATE CAST on DOMAINs

  http://archives.postgresql.org/pgsql-hackers/2006-05/msg00072.php http://archives.postgresql.org/pgsql-hackers/2006-09/msg01681.php

• Add support for SQL-standard GENERATED/IDENTITY columns

  http://archives.postgresql.org/pgsql-hackers/2006-07/msg00543.php http://archives.postgresql.org/pgsql-hackers/2006-08/msg00038.php http://archives.postgresql.org/pgsql-hackers/2007-05/msg00344.php http://archives.postgresql.org/pgsql-patches/2007-05/msg00076.php http://archives.postgresql.org/pgsql-hackers/2008-02/msg00604.php

• Improve XML support

  http://developer.postgresql.org/index.php/XML_Support

• Consider placing all sequences in a single table, or create a system view

  http://archives.postgresql.org/pgsql-hackers/2008-03/msg00008.php

• Allow the UUID type to accept non-standard formats

  http://archives.postgresql.org/pgsql-hackers/2008-02/msg01214.php

• Dates and Times
  • Allow infinite dates and intervals just like infinite timestamps
  • Merge hardwired timezone names with the TZ database; allow either kind everywhere a TZ name is currently taken
  • Allow TIMESTAMP WITH TIME ZONE to store the original timezone information, either zone name or offset from UTC [timezone]

    If the TIMESTAMP value is stored with a time zone name, interval computations should adjust based on the time zone rules.

  • Fix SELECT '0.01 years'::interval, '0.01 months'::interval
  • Add a GUC variable to allow output of interval values in ISO8601 format
  • Have timestamp subtraction not call justify_hours()?

    http://archives.postgresql.org/pgsql-sql/2006-10/msg00059.php

  • Improve timestamptz subtraction to be DST-aware

    Currently subtracting one date from another that crosses a daylight savings time adjustment can return '1 day 1 hour', but adding that back to the first date returns a time one hour in the future. This is caused by the adjustment of '25 hours' to '1 day 1 hour', and '1 day' is the same time the next day, even if daylight savings adjustments are involved.

  • Fix interval display to support values exceeding 2^31 hours
  • Add overflow checking to timestamp and interval arithmetic
  • Extend timezone code to allow 64-bit values so we can represent years beyond 2038

    http://archives.postgresql.org/pgsql-hackers/2006-09/msg01363.php

  • Use LC_TIME for localized weekday/month names, rather than LC_MESSAGES

    http://archives.postgresql.org/pgsql-hackers/2006-11/msg00390.php

  • Add ISO INTERVAL handling

http://archives.postgresql.org/pgsql-hackers/2006-01/msg00250.php
http://archives.postgresql.org/pgsql-bugs/2006-04/msg00248.php

• Support ISO INTERVAL syntax if units cannot be determined from the string, and are supplied after the string

  The SQL standard states that the units after the string specify the units of the string, e.g. INTERVAL '2' MINUTE should return '00:02:00'. The current behavior has the units restrict the interval value to the specified unit or unit range, INTERVAL '70' SECOND returns '00:00:10'.

  For syntax that isn't uniquely ISO or PG syntax, like '1' or '1:30', treat as ISO if there is a range specification clause, and as PG if there no clause is present, e.g. interpret '1:30' MINUTE TO SECOND as '1 minute 30 seconds', and interpret '1:30' as '1 hour, 30 minutes'.

  This makes common cases like SELECT INTERVAL '1' MONTH SQL-standard results. The SQL standard supports a limited number of unit combinations and doesn't support unit names in the string. The PostgreSQL syntax is more flexible in the range of units supported, e.g. PostgreSQL supports '1 year 1 hour', while the SQL standard does not.

• Add support for year-month syntax, INTERVAL '50-6' YEAR TO MONTH
• Interpret INTERVAL '1 year' MONTH as CAST (INTERVAL '1 year' AS INTERVAL MONTH), and this should return '12 months'
• Round or truncate values to the requested precision, e.g. INTERVAL '11 months' AS YEAR should return one or zero
• Support precision, CREATE TABLE foo (a INTERVAL MONTH(3))
  • Arrays
    • Delay resolution of array expression's data type so assignment coercion can be performed on empty array expressions
    • Add support for arrays of domains

      http://archives.postgresql.org/pgsql-patches/2007-05/msg00114.php

    • Allow single-byte header storage for array elements
  • Binary Data
    • Improve vacuum of large objects, like contrib/vacuumlo?
    • Add security checking for large objects
    • Auto-delete large objects when referencing row is deleted

      contrib/lo offers this functionality.

    • Allow read/write into TOAST values like large objects

      This requires the TOAST column to be stored EXTERNAL.

    • Add API for 64-bit large object access

      http://archives.postgresql.org/pgsql-hackers/2005-09/msg00781.php

  • MONEY data type
    • Add locale-aware MONEY type, and support multiple currencies

      http://archives.postgresql.org/pgsql-general/2005-08/msg01432.php http://archives.postgresql.org/pgsql-hackers/2007-03/msg01181.php

    • MONEY dumps in a locale-specific format making it difficult to restore to a system with a different locale
    • Allow MONEY to be easily cast to/from other numeric data types

Functions

• Allow INET subnet tests using non-constants to be indexed
• Allow to_date() and to_timestamp() accept localized month names
• Fix to_date()-related functions to consistently issue errors

  http://archives.postgresql.org/pgsql-hackers/2007-02/msg00915.php

• Add missing parameter handling in to_char()

  http://archives.postgresql.org/pgsql-hackers/2005-12/msg00948.php

• Allow substring/replace() to get/set bit values
• Allow to_char() on interval values to accumulate the highest unit requested

  Some special format flag would be required to request such accumulation. Such functionality could also be added to EXTRACT. Prevent accumulation that crosses the month/day boundary because of the uneven number of days in a month.

  • to_char(INTERVAL '1 hour 5 minutes', 'MI') => 65
  • to_char(INTERVAL '43 hours 20 minutes', 'MI' ) => 2600
  • to_char(INTERVAL '43 hours 20 minutes', 'WK:DD:HR:MI') => 0:1:19:20
  • to_char(INTERVAL '3 years 5 months','MM') => 41
• Implement inlining of set-returning functions defined in SQL
• Allow SQL-language functions to return results from RETURNING queries

  http://archives.postgresql.org/pgsql-hackers/2006-10/msg00665.php

• Allow SQL-language functions to reference parameters by parameter name

  Currently SQL-language functions can only refer to dollar parameters, e.g. $1

• Add SPI_gettypmod() to return the typemod for a TupleDesc
• Enforce typmod for function inputs, function results and parameters for spi_prepare'd statements called from PLs

  http://archives.postgresql.org/pgsql-hackers/2007-01/msg01403.php

• Allow holdable cursors in SPI
• Tighten function permission checks

  http://archives.postgresql.org/pgsql-hackers/2006-12/msg00568.php

• Fix IS OF so it matches the ISO specification, and add documentation

  http://archives.postgresql.org/pgsql-patches/2003-08/msg00060.php http://archives.postgresql.org/pgsql-hackers/2007-02/msg00060.php

• Add missing operators for geometric data types

  Some geometric types do not have the full suite of geometric operators, e.g. box @> point

• Implement Boyer-Moore searching in strpos()

  http://archives.postgresql.org/pgsql-patches/2007-08/msg00012.php

• Prevent malicious functions from being executed with the permissions of unsuspecting users

  Index functions are safe, so VACUUM and ANALYZE are safe too. Triggers, CHECK and DEFAULT expressions, and rules are still vulnerable. http://archives.postgresql.org/pgsql-hackers/2008-01/msg00268.php

• Reduce memory usage of aggregates in set returning functions

  http://archives.postgresql.org/pgsql-performance/2008-01/msg00031.php

Multi-Language Support

• Add NCHAR (as distinguished from ordinary varchar),
• Allow locale to be set at database creation

  Currently locale can only be set during initdb. No global tables have locale-aware columns. However, the database template used during database creation might have locale-aware indexes. The indexes would need to be reindexed to match the new locale.

• Allow encoding on a per-column basis optionally using the ICU library:

  Right now only one encoding is allowed per database. [locale]

  http://archives.postgresql.org/pgsql-hackers/2005-03/msg00932.php http://archives.postgresql.org/pgsql-patches/2005-08/msg00309.php http://archives.postgresql.org/pgsql-patches/2006-03/msg00233.php http://archives.postgresql.org/pgsql-hackers/2006-09/msg00662.php

• Add CREATE COLLATE? [locale]
• Support multiple simultaneous character sets, per SQL92
• Improve UTF8 combined character handling?
• Add octet_length_server() and octet_length_client()
• Make octet_length_client() the same as octet_length()?
• Fix problems with wrong runtime encoding conversion for NLS message files
• Add URL to more complete multi-byte regression tests

  http://archives.postgresql.org/pgsql-hackers/2005-07/msg00272.php

• Fix ILIKE and regular expressions to handle case insensitivity properly in multibyte encodings

  http://archives.postgresql.org/pgsql-bugs/2005-10/msg00001.php http://archives.postgresql.org/pgsql-patches/2005-11/msg00173.php

• Set client encoding based on the client operating system encoding

  Currently client_encoding is set in postgresql.conf, which defaults to the server encoding. http://archives.postgresql.org/pgsql-hackers/2006-08/msg01696.php

Views / Rules

• Automatically create rules on views so they are updateable, per SQL99

  We can only auto-create rules for simple views. For more complex cases users will still have to write rules manually.

  http://archives.postgresql.org/pgsql-hackers/2006-03/msg00586.php http://archives.postgresql.org/pgsql-patches/2006-08/msg00255.php

• Add the functionality for WITH CHECK OPTION clause of CREATE VIEW
• Allow NOTIFY in rules involving conditionals
• Allow VIEW/RULE recompilation when the underlying tables change

  Another issue is whether underlying table changes should be reflected in the view, e.g. should SELECT * show additional columns if they are added after the view is created.

• Make it possible to use RETURNING together with conditional DO INSTEAD rules, such as for partitioning setups

  http://archives.postgresql.org/pgsql-hackers/2007-09/msg00577.php

• Add the ability to automatically create materialized views

  Right now materialized views require the user to create triggers on the main table to keep the summary table current. SQL syntax should be able to manager the triggers and summary table automatically. A more sophisticated implementation would automatically retrieve from the summary table when the main table is referenced, if possible.

SQL Commands

• Add CORRESPONDING BY to UNION/INTERSECT/EXCEPT
• Add ROLLUP, CUBE, GROUPING SETS options to GROUP BY
• %Allow SET CONSTRAINTS to be qualified by schema/table name
• %Add a separate TRUNCATE permission

  Currently only the owner can TRUNCATE a table because triggers are not called, and the table is locked in exclusive mode.

• Allow PREPARE of cursors
• Allow finer control over the caching of prepared query plans

  Currently queries prepared via the libpq API are planned on first execute using the supplied parameters --- allow SQL PREPARE to do the same. Also, allow control over replanning prepared queries either manually or automatically when statistics for execute parameters differ dramatically from those used during planning.

• Improve logging of prepared transactions recovered during startup

  http://archives.postgresql.org/pgsql-hackers/2006-11/msg00092.php

• Allow LISTEN/NOTIFY to store info in memory rather than tables?

  Currently LISTEN/NOTIFY information is stored in pg_listener. Storing such information in memory would improve performance.

• Add optional textual message to NOTIFY

  This would allow an informational message to be added to the notify message, perhaps indicating the row modified or other custom information.

• Allow multiple identical NOTIFY events to always be communicated to the client, rather than sent as a single notification to the listener
• Add a GUC variable to warn about non-standard SQL usage in queries
• Add SQL-standard MERGE command, typically used to merge two tables [merge]

  This is similar to UPDATE, then for unmatched rows, INSERT. Whether concurrent access allows modifications which could cause row loss is implementation independent.

• Add REPLACE or UPSERT command that does UPDATE, or on failure, INSERT [merge]

  To implement this cleanly requires that the table have a unique index so duplicate checking can be easily performed. It is possible to do it without a unique index if we require the user to LOCK the table before the MERGE.

  http://archives.postgresql.org/pgsql-hackers/2005-11/msg00501.php http://archives.postgresql.org/pgsql-hackers/2005-11/msg00536.php

• Add NOVICE output level for helpful messages like automatic sequence/index creation
• Add GUC to issue notice about statements that use unjoined tables
• Allow EXPLAIN to identify tables that were skipped because of constraint_exclusion
• Allow EXPLAIN output to be more easily processed by scripts, perhaps XML
• Enable standard_conforming_strings
• Make standard_conforming_strings the default in 8.5?

  When this is done, backslash-quote should be prohibited in non-E'' strings because of possible confusion over how such strings treat backslashes. Basically, '' is always safe for a literal single quote, while \' might or might not be based on the backslash handling rules.

• Simplify dropping roles that have objects in several databases
• Allow COMMENT ON to accept an expression rather than just a string
• Allow the count returned by SELECT, etc to be represented as an int64 to allow a higher range of values
• Add SQL99 WITH clause to SELECT
• Add SQL:2003 WITH RECURSIVE (hierarchical) queries to SELECT

  http://archives.postgresql.org/pgsql-hackers/2007-01/msg01375.php http://archives.postgresql.org/pgsql-hackers/2008-02/msg00642.php

• Add DEFAULT .. AS OWNER so permission checks are done as the table owner

  This would be useful for SERIAL nextval() calls and CHECK constraints.

• Allow DISTINCT to work in multiple-argument aggregate calls
• Add column to pg_stat_activity that shows the progress of long-running commands like CREATE INDEX and VACUUM
• Implement SQL:2003 window functions
• Improve failure message when DROP DATABASE is used on a database that has prepared transactions
• Allow INSERT/UPDATE ... RETURNING inside a SELECT 'FROM' clause

  http://archives.postgresql.org/pgsql-general/2006-09/msg00803.php http://archives.postgresql.org/pgsql-hackers/2006-10/msg00693.php

• Increase locking when DROPing objects so dependent objects cannot get dropped while the DROP operation is happening

  http://archives.postgresql.org/pgsql-hackers/2007-01/msg00937.php

• -Allow AS in "SELECT col AS label" to be optional in certain cases
• Allow INSERT ... DELETE ... RETURNING, namely allow the DELETE ... RETURNING to supply values to the INSERT http://archives.postgresql.org/pgsql-hackers/2008-02/thrd2.php#00979
• CREATE
  • Allow CREATE TABLE AS to determine column lengths for complex expressions like SELECT col1 || col2
  • Have WITH CONSTRAINTS also create constraint indexes

    http://archives.postgresql.org/pgsql-patches/2007-04/msg00149.php

  • Have CONSTRAINT cname NOT NULL record the contraint name

    Right now pg_attribute.attnotnull records the NOT NULL status of the column, but does not record the contraint name

• UPDATE
  • Allow UPDATE tab SET ROW (col, ...) = (SELECT...)

    http://archives.postgresql.org/pgsql-hackers/2006-07/msg01306.php http://archives.postgresql.org/pgsql-hackers/2007-03/msg00865.php

  • Research self-referential UPDATEs that see inconsistent row versions in read-committed mode

    http://archives.postgresql.org/pgsql-hackers/2007-05/msg00507.php http://archives.postgresql.org/pgsql-hackers/2007-06/msg00016.php

  • Allow GLOBAL temporary tables to exist as empty by default in all sessions

    http://archives.postgresql.org/pgsql-hackers/2007-07/msg00006.php

• ALTER
  • Have ALTER TABLE RENAME rename SERIAL sequence names

    http://archives.postgresql.org/pgsql-hackers/2008-03/msg00008.php

  • Have ALTER SEQUENCE RENAME rename the sequence name stored in the sequence table

    http://archives.postgresql.org/pgsql-bugs/2007-09/msg00092.php http://archives.postgresql.org/pgsql-bugs/2007-10/msg00007.php http://archives.postgresql.org/pgsql-hackers/2008-03/msg00008.php

  • Add ALTER DOMAIN to modify the underlying data type
  • %Allow ALTER TABLE ... ALTER CONSTRAINT ... RENAME

    http://archives.postgresql.org/pgsql-patches/2006-02/msg00168.php

  • %Allow ALTER TABLE to change constraint deferrability and actions
  • Add missing object types for ALTER ... SET SCHEMA
  • Allow ALTER TABLESPACE to move to different directories
  • Allow databases to be moved to different tablespaces
  • Allow moving system tables to other tablespaces, where possible

    Currently non-global system tables must be in the default database tablespace. Global system tables can never be moved.

  • Prevent parent tables from altering or dropping constraints like CHECK that are inherited by child tables unless CASCADE is used
  • %Prevent child tables from altering or dropping constraints like CHECK that were inherited from the parent table
  • Have ALTER INDEX update the name of a constraint using that index
  • Add ALTER TABLE RENAME CONSTRAINT, update index name also
  • Allow column display reordering by recording a display, storage, and permanent id for every column?

    http://archives.postgresql.org/pgsql-hackers/2006-12/msg00782.php

• CLUSTER
  • Automatically maintain clustering on a table

    This might require some background daemon to maintain clustering during periods of low usage. It might also require tables to be only partially filled for easier reorganization. Another idea would be to create a merged heap/index data file so an index lookup would automatically access the heap data too. A third idea would be to store heap rows in hashed groups, perhaps using a user-supplied hash function. http://archives.postgresql.org/pgsql-performance/2004-08/msg00349.php

  • %Add default clustering to system tables

    To do this, determine the ideal cluster index for each system table and set the cluster setting during initdb.

  • %Add VERBOSE option to report tables as they are processed, like VACUUM VERBOSE
• COPY
  • Allow COPY to report error lines and continue

    This requires the use of a savepoint before each COPY line is processed, with ROLLBACK on COPY failure.

  • Allow COPY on a newly-created table to skip WAL logging

    On crash recovery, the table involved in the COPY would be removed or have its heap and index files truncated. One issue is that no other backend should be able to add to the table at the same time, which is something that is currently allowed. This currently is done if the table is created inside the same transaction block as the COPY because no other backends can see the table.

  • Consider using a ring buffer for COPY FROM

    http://archives.postgresql.org/pgsql-patches/2008-02/msg00140.php http://archives.postgresql.org/pgsql-hackers/2008-02/msg01080.php

  • Allow COPY FROM to create index entries in bulk

    http://archives.postgresql.org/pgsql-hackers/2008-02/msg00811.php

• GRANT/REVOKE
  • Allow column-level privileges
  • %Allow GRANT/REVOKE permissions to be applied to all schema objects with one command

    The proposed syntax is:

    GRANT SELECT ON ALL TABLES IN public TO phpuser; GRANT SELECT ON NEW TABLES IN public TO phpuser;

  • Allow GRANT/REVOKE permissions to be inherited by objects based on schema permissions
  • Allow SERIAL sequences to inherit permissions from the base table?
• CURSOR
  • Prevent DROP TABLE from dropping a row referenced by its own open cursor?
• INSERT
  • Allow INSERT/UPDATE of the system-generated oid value for a row
  • In rules, allow VALUES() to contain a mixture of 'old' and 'new' references
• SHOW/SET
  • Add SET PERFORMANCE_TIPS option to suggest INDEX, VACUUM, VACUUM ANALYZE, and CLUSTER

Referential Integrity

• Add MATCH PARTIAL referential integrity
• Change foreign key constraint for array -> element to mean element in array?
• Fix problem when cascading referential triggers make changes on cascaded tables, seeing the tables in an intermediate state

  http://archives.postgresql.org/pgsql-hackers/2005-09/msg00174.php http://archives.postgresql.org/pgsql-hackers/2005-09/msg00174.php

• Allow DEFERRABLE and end-of-statement UNIQUE constraints?

  This would allow UPDATE tab SET col = col + 1 to work if col has a unique index. Currently, uniqueness checks are done while the command is being executed, rather than at the end of the statement or transaction. http://people.planetpostgresql.org/greg/index.php?/archives/2006/06/10.html http://archives.postgresql.org/pgsql-hackers/2006-09/msg01458.php

Server-Side Languages

• PL/pgSQL
  • Fix RENAME to work on variables other than OLD/NEW

    http://archives.postgresql.org/pgsql-hackers/2002-03/msg00591.php http://archives.postgresql.org/pgsql-hackers/2007-01/msg01615.php http://archives.postgresql.org/pgsql-hackers/2007-01/msg01587.php

  • Allow function parameters to be passed by name, get_employee_salary(12345 AS emp_id, 2001 AS tax_year)
  • Allow handling of %TYPE arrays, e.g. tab.col%TYPE[]
  • Allow listing of record column names, and access to record columns via variables, e.g. columns := r.(*), tval2 := r.(colname)

    http://archives.postgresql.org/pgsql-patches/2005-07/msg00458.php http://archives.postgresql.org/pgsql-patches/2006-05/msg00302.php http://archives.postgresql.org/pgsql-patches/2006-06/msg00031.php

  • Add support for SCROLL cursors
  • Add support for WITH HOLD cursors
  • Allow row and record variables to be set to NULL constants, and allow NULL tests on such variables

    Because a row is not scalar, do not allow assignment from NULL-valued scalars.

    http://archives.postgresql.org/pgsql-hackers/2006-10/msg00070.php

• Other
  • Add table function support to pltcl, plpythonu
  • Add support for polymorphic arguments and return types to languages other than PL/PgSQL
  • Add capability to create and call PROCEDURES
  • Add support for OUT and INOUT parameters to languages other than PL/PgSQL
  • Add PL/PythonU tracebacks

    http://archives.postgresql.org/pgsql-patches/2006-02/msg00288.php

  • Allow data to be passed in native language formats, rather than only text

    http://archives.postgresql.org/pgsql-hackers/2007-05/msg00289.php

Clients

• Have pg_ctl look at PGHOST in case it is a socket directory?
• Allow pg_ctl to work properly with configuration files located outside the PGDATA directory

  pg_ctl can not read the pid file because it isn't located in the config directory but in the PGDATA directory. The solution is to allow pg_ctl to read and understand postgresql.conf to find the data_directory value.

• psql
  • Have psql show current values for a sequence
  • Move psql backslash database information into the backend, use mnemonic commands? [psql]

    This would allow non-psql clients to pull the same information out of the database as psql.

  • Fix psql's \d commands more consistent

    http://archives.postgresql.org/pgsql-hackers/2004-11/msg00014.php http://archives.postgresql.org/pgsql-hackers/2004-11/msg00014.php

  • Consistently display privilege information for all objects in psql
  • Add auto-expanded mode so expanded output is used if the row length is wider than the screen width.

    Consider using auto-expanded mode for backslash commands like \df+.

  • Prevent tab completion of SET TRANSACTION from querying the database and therefore preventing the transaction isolation level from being set.

    Currently SET <tab> causes a database lookup to check all supported session variables. This query causes problems because setting the transaction isolation level must be the first statement of a transaction.

  • Add a \set variable to control whether \s displays line numbers

    Another option is to add \# which lists line numbers, and allows command execution.

    http://archives.postgresql.org/pgsql-hackers/2006-12/msg00255.php

  • Prevent escape string warnings when object names have backslashes

    http://archives.postgresql.org/pgsql-hackers/2008-01/msg00227.php

• pg_dump / pg_restore
  • %Add dumping of comments on index columns and composite type columns
  • %Add full object name to the tag field. eg. for operators we need '=(integer, integer)', instead of just '='.
  • Add pg_dumpall custom format dumps?
  • Allow selection of individual object(s) of all types, not just tables
  • In a selective dump, allow dumping of an object and all its dependencies
  • Add options like pg_restore -l and -L to pg_dump
  • Stop dumping CASCADE on DROP TYPE commands in clean mode
  • Allow pg_dump --clean to drop roles that own objects or have privileges
  • Change pg_dump so that a comment on the dumped database is applied to the loaded database, even if the database has a different name. This will require new backend syntax, perhaps COMMENT ON CURRENT DATABASE.
  • Remove unnecessary function pointer abstractions in pg_dump source code
  • Allow pg_dump to utilize multiple CPUs and I/O channels by dumping multiple objects simultaneously

    The difficulty with this is getting multiple dump processes to produce a single dump output file. It also would require several sessions to share the same snapshot. http://archives.postgresql.org/pgsql-hackers/2008-02/msg00205.php

  • Allow pg_restore to utilize multiple CPUs and I/O channels by restoring multiple objects simultaneously

    This might require a pg_restore flag to indicate how many simultaneous operations should be performed. Only pg_dump's -Fc format has the necessary dependency information. http://archives.postgresql.org/pgsql-hackers/2008-02/msg00963.php

  • To better utilize resources, allow pg_restore to check foreign keys simultaneously, where possible
  • Allow pg_restore to create all indexes of a table concurrently, via a single heap scan

    This requires a pg_dump -Fc file because that format contains the required dependency information. http://archives.postgresql.org/pgsql-general/2007-05/msg01274.php

  • Allow pg_restore to load different parts of the COPY data simultaneously
• ecpg
  • Docs

    Document differences between ecpg and the SQL standard and information about the Informix-compatibility module.

  • Solve cardinality > 1 for input descriptors / variables?
  • Add a semantic check level, e.g. check if a table really exists
  • fix handling of DB attributes that are arrays
  • Use backend PREPARE/EXECUTE facility for ecpg where possible
  • Implement SQLDA
  • Fix nested C comments
  • %sqlwarn[6] should be 'W' if the PRECISION or SCALE value specified
  • Make SET CONNECTION thread-aware, non-standard?
  • Allow multidimensional arrays
  • Add internationalized message strings
  • Implement COPY FROM STDIN
• libpq
  • Add PQescapeIdentifierConn()
  • Prevent PQfnumber() from lowercasing unquoted the column name

    PQfnumber() should never have been doing lowercasing, but historically it has so we need a way to prevent it

  • Allow statement results to be automatically batched to the client

    Currently all statement results are transferred to the libpq client before libpq makes the results available to the application. This feature would allow the application to make use of the first result rows while the rest are transferred, or held on the server waiting for them to be requested by libpq. One complexity is that a statement like SELECT 1/col could error out mid-way through the result set.

  • Consider disallowing multiple queries in PQexec() as an additional barrier to SQL injection attacks

    http://archives.postgresql.org/pgsql-hackers/2007-01/msg00184.php

  • Add PQexecf() that allows complex parameter substitution

    http://archives.postgresql.org/pgsql-hackers/2007-03/msg01803.php

Triggers

• Add deferred trigger queue file

  Right now all deferred trigger information is stored in backend memory. This could exhaust memory for very large trigger queues. This item involves dumping large queues into files.

• Allow triggers to be disabled in only the current session.

  This is currently possible by starting a multi-statement transaction, modifying the system tables, performing the desired SQL, restoring the system tables, and committing the transaction. ALTER TABLE ... TRIGGER requires a table lock so it is not ideal for this usage.

• With disabled triggers, allow pg_dump to use ALTER TABLE ADD FOREIGN KEY

  If the dump is known to be valid, allow foreign keys to be added without revalidating the data.

• Allow statement-level triggers to access modified rows
• Support triggers on columns

  http://archives.postgresql.org/pgsql-patches/2005-07/msg00107.php

• Allow AFTER triggers on system tables

  System tables are modified in many places in the backend without going through the executor and therefore not causing triggers to fire. To complete this item, the functions that modify system tables will have to fire triggers.

• Tighten trigger permission checks

  http://archives.postgresql.org/pgsql-hackers/2006-12/msg00564.php

• Allow BEFORE INSERT triggers on views

  http://archives.postgresql.org/pgsql-general/2007-02/msg01466.php

• Add ability to trigger on TRUNCATE

  http://archives.postgresql.org/pgsql-sql/2008-01/msg00050.php

Indexes

• Add UNIQUE capability to non-btree indexes
• Prevent index uniqueness checks when UPDATE does not modify the column

  Uniqueness (index) checks are done when updating a column even if the column is not modified by the UPDATE.

• Allow the creation of on-disk bitmap indexes which can be quickly combined with other bitmap indexes

  Such indexes could be more compact if there are only a few distinct values. Such indexes can also be compressed. Keeping such indexes updated can be costly.

  http://archives.postgresql.org/pgsql-patches/2005-07/msg00512.php http://archives.postgresql.org/pgsql-hackers/2006-12/msg01107.php

• Allow accurate statistics to be collected on indexes with more than one column or expression indexes, perhaps using per-index statistics

  http://archives.postgresql.org/pgsql-performance/2006-10/msg00222.php http://archives.postgresql.org/pgsql-hackers/2007-03/msg01131.php

• Consider compressing indexes by storing key values duplicated in several rows as a single index entry
• Add REINDEX CONCURRENTLY, like CREATE INDEX CONCURRENTLY

  This is difficult because you must upgrade to an exclusive table lock to replace the existing index file. CREATE INDEX CONCURRENTLY does not have this complication. This would allow index compaction without downtime.

• Allow multiple indexes to be created concurrently, ideally via a single heap scan, and have pg_restore use it
• Inheritance
  • Allow inherited tables to inherit indexes, UNIQUE constraints, and primary/foreign keys
  • Honor UNIQUE INDEX on base column in INSERTs/UPDATEs on inherited table, e.g. INSERT INTO inherit_table (unique_index_col) VALUES (dup) should fail

    The main difficulty with this item is the problem of creating an index that can span multiple tables.

  • Allow SELECT ... FOR UPDATE on inherited tables
• GIST
  • Add more GIST index support for geometric data types
  • Allow GIST indexes to create certain complex index types, like digital trees (see Aoki)
• Hash

http://archives.postgresql.org/pgsql-hackers/2007-09/msg00051.php

• Pack hash index buckets onto disk pages more efficiently

  Currently only one hash bucket can be stored on a page. Ideally several hash buckets could be stored on a single page and greater granularity used for the hash algorithm.

  http://archives.postgresql.org/pgsql-hackers/2004-06/msg00168.php

• Consider sorting hash buckets so entries can be found using a binary search, rather than a linear scan
• In hash indexes, consider storing the hash value with or instead of the key itself
• Add WAL logging for crash recovery
• Allow multi-column hash indexes
• During index creation, pre-sort the tuples to improve build speed

  http://archives.postgresql.org/pgsql-hackers/2007-03/msg01199.php

Fsync

• Determine optimal fdatasync/fsync, O_SYNC/O_DSYNC options

  Ideally this requires a separate test program that can be run at initdb time or optionally later. Consider O_SYNC when O_DIRECT exists.

• Add program to test if fsync has a delay compared to non-fsync

Cache Usage

• Speed up COUNT(*)

  We could use a fixed row count and a +/- count to follow MVCC visibility rules, or a single cached value could be used and invalidated if anyone modifies the table. Another idea is to get a count directly from a unique index, but for this to be faster than a sequential scan it must avoid access to the heap to obtain tuple visibility information.

• Provide a way to calculate an "estimated COUNT(*)"

  Perhaps by using the optimizer's cardinality estimates or random sampling.

  http://archives.postgresql.org/pgsql-hackers/2005-11/msg00943.php

• Allow data to be pulled directly from indexes

  Currently indexes do not have enough tuple visibility information to allow data to be pulled from the index without also accessing the heap. One way to allow this is to set a bit on index tuples to indicate if a tuple is currently visible to all transactions when the first valid heap lookup happens. This bit would have to be cleared when a heap tuple is expired.

  Another idea is to maintain a bitmap of heap pages where all rows are visible to all backends, and allow index lookups to reference that bitmap to avoid heap lookups, perhaps the same bitmap we might add someday to determine which heap pages need vacuuming. Frequently accessed bitmaps would have to be stored in shared memory. One 8k page of bitmaps could track 512MB of heap pages.

  A third idea would be for a heap scan to check if all rows are visible and if so set a per-table flag which can be checked by index scans. Any change to the table would have to clear the flag. To detect changes during the heap scan a counter could be set at the start and checked at the end --- if it is the same, the table has not been modified --- any table change would increment the counter.

• Consider automatic caching of statements at various levels:
  • Parsed query tree
  • Query execute plan
  • Query results
• Consider increasing internal areas when shared buffers is increased

  http://archives.postgresql.org/pgsql-hackers/2005-10/msg01419.php

• Consider decreasing the amount of memory used by PrivateRefCount

  http://archives.postgresql.org/pgsql-hackers/2006-11/msg00797.php http://archives.postgresql.org/pgsql-hackers/2007-01/msg00752.php

Vacuum

• Improve speed with indexes

  For large table adjustments during VACUUM FULL, it is faster to cluster or reindex rather than update the index. Also, index updates can bloat the index.

  http://archives.postgresql.org/pgsql-hackers/2007-03/msg00024.php http://archives.postgresql.org/pgsql-performance/2007-05/msg00296.php

• Auto-fill the free space map by scanning the buffer cache or by checking pages written by the background writer

  http://archives.postgresql.org/pgsql-hackers/2006-02/msg01125.php http://archives.postgresql.org/pgsql-hackers/2006-03/msg00011.php

• Create a bitmap of pages that need vacuuming

  Instead of sequentially scanning the entire table, have the background writer or some other process record pages that have expired rows, then VACUUM can look at just those pages rather than the entire table. In the event of a system crash, the bitmap would probably be invalidated. One complexity is that index entries still have to be vacuumed, and doing this without an index scan (by using the heap values to find the index entry) might be slow and unreliable, especially for user-defined index functions.

  http://archives.postgresql.org/pgsql-hackers/2006-12/msg01188.php http://archives.postgresql.org/pgsql-hackers/2007-01/msg00121.php

• Allow FSM to return free space toward the beginning of the heap file, in hopes that empty pages at the end can be truncated by VACUUM
• Allow FSM page return free space based on table clustering, to assist in maintaining clustering?
• Improve dead row detection during multi-statement transactions usage

  http://archives.postgresql.org/pgsql-patches/2007-03/msg00358.php

• Auto-vacuum
  • %Issue log message to suggest VACUUM FULL if a table is nearly empty?
  • Improve control of auto-vacuum

    http://archives.postgresql.org/pgsql-hackers/2006-12/msg00876.php

  • Prevent long-lived temporary tables from causing frozen-xid advancement starvation

    The problem is that autovacuum cannot vacuum them to set frozen xids; only the session that created them can do that. http://archives.postgresql.org/pgsql-general/2007-06/msg01645.php

Locking

• Fix priority ordering of read and write light-weight locks (Neil)

  http://archives.postgresql.org/pgsql-hackers/2004-11/msg00893.php http://archives.postgresql.org/pgsql-hackers/2004-11/msg00905.php

• Fix problem when multiple subtransactions of the same outer transaction hold different types of locks, and one subtransaction aborts

  http://archives.postgresql.org/pgsql-hackers/2006-11/msg01011.php http://archives.postgresql.org/pgsql-hackers/2006-12/msg00001.php http://archives.postgresql.org/pgsql-hackers/2007-02/msg00435.php

• Allow UPDATEs on only non-referential integrity columns not to conflict with referential integrity locks

  http://archives.postgresql.org/pgsql-hackers/2007-02/msg00073.php

• Add idle_in_transaction_timeout GUC so locks are not held for long periods of time
• Improve deadlock detection when a page cleaning lock conflicts with a shared buffer that is pinned

  http://archives.postgresql.org/pgsql-bugs/2008-01/msg00138.php http://archives.postgresql.org/pgsql-hackers/2008-01/msg00873.php http://archives.postgresql.org/pgsql-committers/2008-01/msg00365.php

Startup Time Improvements

• Experiment with multi-threaded backend for backend creation [thread]

  This would prevent the overhead associated with process creation. Most operating systems have trivial process creation time compared to database startup overhead, but a few operating systems (Win32, Solaris) might benefit from threading. Also explore the idea of a single session using multiple threads to execute a statement faster.

Write-Ahead Log

• Eliminate need to write full pages to WAL before page modification [wal]

  Currently, to protect against partial disk page writes, we write full page images to WAL before they are modified so we can correct any partial page writes during recovery. These pages can also be eliminated from point-in-time archive files.

  • When off, write CRC to WAL and check file system blocks on recovery

    If CRC check fails during recovery, remember the page in case a later CRC for that page properly matches.

  • Write full pages during file system write and not when the page is modified in the buffer cache

    This allows most full page writes to happen in the background writer. It might cause problems for applying WAL on recovery into a partially-written page, but later the full page will be replaced from WAL.

• Allow WAL traffic to be streamed to another server for stand-by replication
• Reduce WAL traffic so only modified values are written rather than entire rows

  http://archives.postgresql.org/pgsql-hackers/2007-03/msg01589.php

• Allow WAL information to recover corrupted pg_controldata

  http://archives.postgresql.org/pgsql-patches/2006-06/msg00025.php

• Find a way to reduce rotational delay when repeatedly writing last WAL page

  Currently fsync of WAL requires the disk platter to perform a full rotation to fsync again. One idea is to write the WAL to different offsets that might reduce the rotational delay.

• Allow WAL logging to be turned off for a table, but the table might be dropped or truncated during crash recovery [walcontrol]

  Allow tables to bypass WAL writes and just fsync() dirty pages on commit. This should be implemented using ALTER TABLE, e.g. ALTER TABLE PERSISTENCE [ DROP | TRUNCATE | DEFAULT ]. Tables using non-default logging should not use referential integrity with default-logging tables. A table without dirty buffers during a crash could perhaps avoid the drop/truncate.

• Allow WAL logging to be turned off for a table, but the table would avoid being truncated/dropped [walcontrol]

  To do this, only a single writer can modify the table, and writes must happen only on new pages so the new pages can be removed during crash recovery. Readers can continue accessing the table. Such tables probably cannot have indexes. One complexity is the handling of indexes on TOAST tables.

• Speed WAL recovery by allowing more than one page to be prefetched

  This involves having a separate process that can be told which pages the recovery process will need in the near future. http://archives.postgresql.org/pgsql-hackers/2008-02/msg01279.php

Optimizer / Executor

• Improve selectivity functions for geometric operators
• Precompile SQL functions to avoid overhead
• Create utility to compute accurate random_page_cost value
• Improve ability to display optimizer analysis using OPTIMIZER_DEBUG
• Have EXPLAIN ANALYZE issue NOTICE messages when the estimated and actual row counts differ by a specified percentage
• Consider using hash buckets to do DISTINCT, rather than sorting

  This would be beneficial when there are few distinct values. This is already used by GROUP BY.

• Log statements where the optimizer row estimates were dramatically different from the number of rows actually found?
• Consider compressed annealing to search for query plans

  This might replace GEQO, http://sixdemonbag.org/Djinni.

• Improve merge join performance by allowing mark/restore of tuple sources

  http://archives.postgresql.org/pgsql-hackers/2007-01/msg00096.php

Miscellaneous Performance

• Do async I/O for faster random read-ahead of data

  Async I/O allows multiple I/O requests to be sent to the disk with results coming back asynchronously.

  http://archives.postgresql.org/pgsql-hackers/2006-10/msg00820.php http://archives.postgresql.org/pgsql-hackers/2007-12/msg00027.php

• Use mmap() rather than SYSV shared memory or to write WAL files?

  This would remove the requirement for SYSV SHM but would introduce portability issues. Anonymous mmap (or mmap to /dev/zero) is required to prevent I/O overhead.

• Consider mmap()'ing files into a backend?

  Doing I/O to large tables would consume a lot of address space or require frequent mapping/unmapping. Extending the file also causes mapping problems that might require mapping only individual pages, leading to thousands of mappings. Another problem is that there is no way to prevent I/O to disk from the dirty shared buffers so changes could hit disk before WAL is written.

• Add a script to ask system configuration questions and tune postgresql.conf
• Consider ways of storing rows more compactly on disk
  • Reduce the row header size?
  • Consider reducing on-disk varlena length from four bytes to two because a heap row cannot be more than 64k in length
• Consider increasing NUM_CLOG_BUFFERS
• Consider having the background writer update the transaction status hint bits before writing out the page

  Implementing this requires the background writer to have access to system catalogs and the transaction status log.

• Allow user configuration of TOAST thresholds

  http://archives.postgresql.org/pgsql-hackers/2007-02/msg00213.php

• Allow configuration of backend priorities via the operating system

  Though backend priorities make priority inversion during lock waits possible, research shows that this is not a huge problem.

  http://archives.postgresql.org/pgsql-general/2007-02/msg00493.php

• Experiment with multi-threaded backend better resource utilization

  This would allow a single query to make use of multiple CPU's or multiple I/O channels simultaneously. One idea is to create a background reader that can pre-fetch sequential and index scan pages needed by other backends. This could be expanded to allow concurrent reads from multiple devices in a partitioned table.

Source Code

• Add use of 'const' for variables in source tree
• Move some things from contrib into main tree
• %Remove warnings created by -Wcast-align
• Move platform-specific ps status display info from ps_status.c to ports
• Add optional CRC checksum to heap and index pages
• Improve documentation to build only interfaces (Marc)
• Remove or relicense modules that are not under the BSD license, if possible
• Acquire lock on a relation before building a relcache entry for it
• Allow cross-compiling by generating the zic database on the target system
• Improve NLS maintenance of libpgport messages linked onto applications
• Clean up casting in contrib/isn

  http://archives.postgresql.org/pgsql-hackers/2006-11/msg00245.php

• Use UTF8 encoding for NLS messages so all server encodings can read them properly
• Update Bonjour to work with newer cross-platform SDK

  http://archives.postgresql.org/pgsql-hackers/2006-09/msg02238.php http://archives.postgresql.org/pgsql-patches/2006-10/msg00048.php

• Consider detoasting keys before sorting
• Consider GnuTLS if OpenSSL license becomes a problem

  http://archives.postgresql.org/pgsql-patches/2006-05/msg00040.php http://archives.postgresql.org/pgsql-hackers/2006-12/msg01213.php

• Consider changing documentation format from SGML to XML

  http://archives.postgresql.org/pgsql-docs/2006-12/msg00152.php

• Consider making NAMEDATALEN more configurable in future releases
• Update our code to handle 64-bit timezone files to match the zic source code, which now uses them
• Have configure choose integer datetimes by default

  http://archives.postgresql.org/pgsql-patches/2007-05/msg00046.php

• Support scoped IPv6 addresses

  http://archives.postgresql.org/pgsql-bugs/2007-05/msg00111.php

• Consider allowing 64-bit integers and floats to be passed by value on 64-bit platforms

  Also change 32-bit floats (float4) to be passed by value at the same time.

• Win32
  • Remove configure.in check for link failure when cause is found
  • Remove readdir() errno patch when runtime/mingwex/dirent.c rev 1.4 is released
  • Remove psql newline patch when we find out why mingw outputs an extra newline
  • Allow psql to use readline once non-US code pages work with backslashes
  • Fix problem with shared memory on the Win32 Terminal Server
  • Improve signal handling

    http://archives.postgresql.org/pgsql-patches/2005-06/msg00027.php

• Wire Protocol Changes
  • Allow dynamic character set handling
  • Add decoded type, length, precision
  • Use compression?
  • Update clients to use data types, typmod, schema.table.column names of result sets using new statement protocol

Exotic Features

• Add pre-parsing phase that converts non-ISO syntax to supported syntax

  This could allow SQL written for other databases to run without modification.

• Allow plug-in modules to emulate features from other databases
• Add features of Oracle-style packages (Pavel)

  A package would be a schema with session-local variables, public/private functions, and initialization functions. It is also possible to implement these capabilities in any schema and not use a separate "packages" syntax at all.

  http://archives.postgresql.org/pgsql-hackers/2006-08/msg00384.php

• Consider allowing control of upper/lower case folding of unquoted identifiers

  http://archives.postgresql.org/pgsql-hackers/2004-04/msg00818.php http://archives.postgresql.org/pgsql-hackers/2006-10/msg01527.php

• Add autonomous transactions

  http://archives.postgresql.org/pgsql-hackers/2008-01/msg00893.php

Features We Do Not Want

• All backends running as threads in a single process (not wanted)

  This eliminates the process protection we get from the current setup. Thread creation is usually the same overhead as process creation on modern systems, so it seems unwise to use a pure threaded model.

• Optimizer hints (not wanted)

  Optimizer hints are used to work around problems in the optimizer. We would rather have the problems reported and fixed.

  http://archives.postgresql.org/pgsql-hackers/2006-08/msg00506.php http://archives.postgresql.org/pgsql-hackers/2006-10/msg00517.php http://archives.postgresql.org/pgsql-hackers/2006-10/msg00663.php

  Because we support postfix operators, it isn't possible to make AS optional and continue to use bison. http://archives.postgresql.org/pgsql-hackers/2003-04/msg00436.php

  http://archives.postgresql.org/pgsql-sql/2006-08/msg00164.php

• Embedded server (not wanted)

  While PostgreSQL clients runs fine in limited-resource environments, the server requires multiple processes and a stable pool of resources to run reliabily and efficiently. Stripping down the PostgreSQL server to run in the same process address space as the client application would add too much complexity and failure cases.