From 3d22596fe79328bfd850f57df2e3130b3bba4d82 Mon Sep 17 00:00:00 2001
From: "Thomas G. Lockhart" <lockhart@fourpalms.org>
Date: Fri, 6 Nov 1998 15:04:31 +0000
Subject: [PATCH] August version of docs superceded by sgml version from Jan.

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-                                PL/pgSQL
-             A procedural language for the PostgreSQL RDBMS
-
-                      Jan Wieck <jwieck@debis.com>
-
-
-
-    Preface
-
-        PL/pgSQL  is  a procedural language based on SQL designed for
-        the PostgreSQL database system.
-
-        The extensibility features of PostgreSQL are mostly based  on
-        the  ability  to  define  functions  for  various operations.
-        Functions could have been written in PostgreSQL's SQL dialect
-        or  in the C programming language. Functions written in C are
-        compiled into a shared object  and  loaded  by  the  database
-        backend  process  on  demand.   Also  the trigger features of
-        PostgreSQL are based on functions but required the use of the
-        C language.
-
-        Since  version  6.3  PostgreSQL  supports  the  definition of
-        procedural languages. In the case of a  function  or  trigger
-        procedure  defined in a procedural language, the database has
-        no builtin knowlege how to  interpret  the  functions  source
-        text. Instead, the function and trigger calls are passed into
-        a handler that  knows  the  details  of  the  language.   The
-        handler  itself  is  a function compiled into a shared object
-        and loaded on demand.
-
-
-    Overview
-
-        The PL/pgSQL language is case insensitive. All  keywords  and
-        identifiers can be used in upper-/lowercase mixed.
-
-        PL/pgSQL is a block oriented language. A block is defined as
-
-            [<<label>>]
-            [DECLARE
-                -- declarations]
-            BEGIN
-                -- statements
-            END;
-
-        There  can  be  any  number  of  subblocks  in the statements
-        section of a block. Subblocks can be used to  hide  variables
-        from  outside a block of statements (see Scope and visability
-        below). The variables declared in  the  declarations  section
-        preceding  a  block  are  initialized to their default values
-        every time the block is entered, not only once  per  function
-        call.
-
-        It is important not to misunderstand the meaning of BEGIN/END
-        for grouping statements in PL/pgSQL and the database commands
-        for  transaction  control.  Functions  or  trigger procedures
-        cannot  start   or   commit   transactions   and   PostgreSQL
-        transactions cannot have subtransactions.
-
-
-    Comments
-
-        There  are  two  types of comments in PL/pgSQL. A double dash
-        '--' starts a comment that extends to the end of the line.  A
-        '/*'  starts  a  block comment that extends to the next '*/'.
-        Block comments cannot be nested, but double dash comments can
-        be  enclosed  into a block comment and double dashes can hide
-        '/*' and '*/'.
-
-
-    Declarations
-
-        All variables, rows and records  used  in  a  block  or  it's
-        subblocks must be declared in the declarations section of the
-        block except for the loop variable of a  FOR  loop  iterating
-        over  a range of integer values.  The parameters given to the
-        function  are   automatically   declared   with   the   usual
-        identifiers $n.  The declarations have the following syntax:
-
-            <name> [CONSTANT] <type> [NOT NULL]
-                                     [DEFAULT | := <value>];
-
-                Declares  a  variable  of  the specified type. If the
-                variable is declared as CONSTANT, the value cannot be
-                changed. If NOT NULL is specified, an assignment of a
-                NULL value results in  a  runtime  error.  Since  the
-                default  value  of  a variable is the SQL NULL value,
-                all variables declared as NOT NULL must also  have  a
-                default value.
-
-                The default value is evaluated at the actual function
-                call. So  assigning  'now'  to  an  abstime  variable
-                causes  the  variable  to have the time of the actual
-                function call, not when  the  function  was  compiled
-                (during  it's  first  call  since the lifetime of the
-                database connection).
-
-            <name> <class>%ROWTYPE;
-
-                Declares a row with the structure of the given class.
-                Class  must  be an existing table- or viewname of the
-                database. The fields of the row are accessed  in  the
-                dot  notation.  Parameters  to  a  procedure could be
-                tuple  types.  In   that   case   the   corresponding
-                identifier  $n  will  be  a  rowtype.  Only  the user
-                attributes of a tuple  are  accessible  in  the  row.
-                There  must  be no whitespaces between the classname,
-                the percent and the ROWTYPE keyword.
-
-                The  fields  of  the  rowtype  inherit   the   tables
-                fieldsizes  for  char()  etc.   data types (atttypmod
-                from pg_attribute).
-
-            <name> RECORD;
-
-                Records are similar to rowtypes,  but  they  have  no
-                predefined  structure They are used in selections and
-                FOR loops to hold one actual database  tuple  from  a
-                select operation. One and the same record can be used
-                in different selections. Accessing  a  record  or  an
-                attempt  to  assign  a  value  to a record field when
-                there's no actual tuple in it results  in  a  runtime
-                error.
-
-                The  new  and old tuples in triggers are given to the
-                trigger procedure  as  records.  This  is  necessary,
-                because  under  PostgreSQL  one  and the same trigger
-                procedure can handle  trigger  events  for  different
-                tables.
-
-            <name> ALIAS FOR $n;
-
-                For  better  readability of the code it's possible to
-                define an alias for a  positional  parameter  to  the
-                function.
-
-            RENAME <oldname> TO <newname>;
-
-                Change  the  name  of  a variable, record or rowtype.
-                This is useful if new or old should be referenced  by
-                another name inside a trigger procedure.
-
-    Datatypes
-
-        The  type of a variable can be any of the existing data types
-        of the database. <type> above is defined as:
-
-                postgesql-basetype
-            or  variable%TYPE
-            or  rowtype.field%TYPE
-            or  class.field%TYPE
-
-        As for the rowtype declaration, there must be no  whitespaces
-        between the classname, the percent and the TYPE keyword.
-
-    Expressions
-
-        All  expressions  used  in  PL/pgSQL statements are processed
-        using the backends executor. Since even  a  constant  looking
-        expression  can  have  a  totally  different  meaning  for  a
-        particular data type (as 'now' for abstime), it is impossible
-        for  the  PL/pgSQL  parser  to  identify real constant values
-        other than the NULL keyword. The expressions are evaluated by
-        internally executing a query
-
-            SELECT <expr>
-
-        over  the  SPI  manager.  In  the  expression,  occurences of
-        variable identifiers are substituted by  parameters  and  the
-        actual  values  from the variables are passed to the executor
-        as query parameters. All the expressions used in  a  PL/pgSQL
-        function are only prepared and saved once.
-
-        If   record  fields  are  used  in  expressions  or  database
-        statements, the data types of the fields  should  not  change
-        between  calls  of  one and the same expression. Keep this in
-        mind when writing trigger procedures that handle  events  for
-        more than one table.
-
-    Statements
-
-        Anything not understood by the parser as specified below will
-        be put into a query and sent down to the database  engine  to
-        execute.   The  resulting  query  should  not return any data
-        (insert, update, delete queries and all utility  statements).
-
-        Assignment
-
-            An  assignment  of a value to a variable or rowtype field
-            is written as:
-
-                <identifier> := <expr>;
-
-            If the expressions result data  type  doesn't  match  the
-            variables  data type, or the variables atttypmod value is
-            known  (as  for  char(20)),  the  result  value  will  be
-            implicitly  casted  by  the  PL/pgSQL  executor using the
-            result  types  output-  and  the  variables  type  input-
-            functions.   Note  that  this could potentially result in
-            runtime errors generated by the types input functions.
-
-            An assignment of a complete selection into  a  record  or
-            rowtype can be done as:
-
-                SELECT expressions INTO <target> FROM fromlist;
-
-            Target  can  be  a record or rowtype variable, or a comma
-            separated list of variables and record/row fields.
-
-            If a rowtype or a variable list is used  as  target,  the
-            selected  values  must exactly match the structure of the
-            target(s) or a runtime error occurs.  The fromlist can be
-            followed  by  any  valid qualification, grouping, sorting
-            etc.
-
-            There is a special condition [NOT] FOUND that can be used
-            immediately  after a SELECT INTO to check if the data has
-            been found.
-
-                SELECT * INTO myrec FROM EMP WHERE empname = myname;
-                IF NOT FOUND THEN
-                    RAISE EXCEPTION 'employee % not found', myname;
-                END IF;
-
-            If the selection returns multiple rows, only the first is
-            moved into the target fields. All others are discarded.
-
-
-        Calling another function
-
-            If  a function should be called, this is normally done by
-            a SELECT query. But there are cases where  someone  isn't
-            interested in the functions result.
-
-                PERFORM querystring;
-
-            executes  a 'SELECT querystring' over the SPI manager and
-            discards the result.
-
-
-        Returning from the function
-
-                RETURN <expr>;
-
-            The function terminates and the value of <expr>  will  be
-            returned  to  the  upper executor.  The return value of a
-            function cannot be undefined.  If control reaches the end
-            of  the  toplevel block of the function without hitting a
-            RETURN statement, a runtime error will occur.
-
-
-        Aborting and messages
-
-            As indicated above there is an RAISE statement  that  can
-            throw messages into the PostgreSQL elog mechanism.
-
-                RAISE level 'format' [, identifier [...]];
-
-            Inside the format, % can be used as a placeholder for the
-            following, comma separated identifiers.  The  identifiers
-            must specify an existing variable or row/record field.
-
-
-        Conditionals
-
-                IF <expr> THEN
-                    -- statements
-                [ELSE
-                    -- statements]
-                END IF;
-
-            The  expression  <expr> must return a value that at least
-            can be casted into a boolean.
-
-
-        Loops
-
-            There are multiple types of loops.
-
-                [<<label>>]
-                LOOP
-                    -- statements
-                END LOOP;
-
-            An unconditional loop that must be terminated  explicitly
-            by  an  EXIT statement. The optional label can be used by
-            EXIT statements of nested loops to specify which level of
-            nesting should be terminated.
-
-                [<<label>>]
-                WHILE <expr> LOOP
-                    -- statements
-                END LOOP;
-
-            A  conditional  loop  that  is  executed  as  long as the
-            evaluation of <expr> returns true.
-
-                [<<label>>]
-                FOR <name> IN [REVERSE] <expr>..<expr> LOOP
-                    -- statements
-                END LOOP.
-
-            A loop that iterates over a range of integer values.  The
-            variable  <name> is automatically created as type integer
-            and exists only inside  the  loop.  The  two  expressions
-            giving  the  lower  and  upper  bound  of  the  range are
-            evaluated only when entering the loop. The iteration step
-            is 1.
-
-                FOR <recname|rowname> IN <select_clause> LOOP
-                    -- statements
-                END LOOP;
-
-            The record or row is assigned all the rows resulting from
-            the select clause and the statements executed  for  each.
-            If  the  loop  is  terminated with an EXIT statement, the
-            last accessed row is still accessible in  the  record  or
-            rowtype.
-
-                EXIT [label] [WHEN <expr>];
-
-            If  no  label given, the innermost loop is terminated and
-            the statement following END LOOP  is  executed  next.  If
-            label is given, it must be the label of the current or an
-            upper level of nested loops or blocks.   Then  the  named
-            loop  or  block  is terminated and control continues with
-            the statement after the loops/blocks corresponding END.
-
-    Trigger procedures
-
-        PL/pgSQL can also be used to define trigger procedures.  They
-        are  created  using  CREATE  FUNCTION  as  a function with no
-        arguments and a return type of opaque.
-
-        There are some PostgreSQL specific details in functions  used
-        as trigger procedures.
-
-        First  they  have  some  special  variables created above the
-        toplevel statement block. These are:
-
-            new (record)
-                The new database tuple on INSERT/UPDATE operations at
-                ROW level.
-
-            old (record)
-                The old database tuple on UPDATE/DELETE operations at
-                ROW level.
-
-            tg_name (type name)
-                The triggers name from pg_trigger.
-
-            tg_when (type text)
-                A string of either 'BEFORE' or 'AFTER'  depending  on
-                the triggers definition.
-
-            tg_level (type text)
-                A  string of either 'ROW' or 'STATEMENT' depending on
-                the triggers definition.
-
-            tg_op (type text)
-                A string of 'INSERT', 'UPDATE'  or  'DELETE'  telling
-                for which operation the trigger is actually fired.
-
-            tg_relid (type oid)
-                The  Oid  of  the  relation  for which the trigger is
-                actually fired.
-
-            tg_relname (type name)
-                The relations name for which the trigger is  actually
-                fired.
-
-            tg_nargs (type integer)
-                The   number   of  arguments  given  to  the  trigger
-                procedure in the CREATE TRIGGER statement.
-
-            tg_argv[] (types text)
-                The arguments from the CREATE TRIGGER statement.  The
-                index  counts  from 0 and can be given as expression.
-                Invalid indices (< 0 or >= tg_nargs) result in a NULL
-                value.
-
-        Second,  they  must  return  either NULL, or a record/rowtype
-        containing exactly the structure of the table the trigger was
-        fired  for.  Triggers  fired  AFTER might allways return NULL
-        with no effect. Triggers  fired  BEFORE  signal  the  trigger
-        manager  to  skip  the  operation  for  this  actual row when
-        returning  NULL.  Otherwise,  the   returned   record/rowtype
-        replaces  the  inserted/updated tuple in the operation. It is
-        possible to replace single values directly in new and  return
-        that, or to build a complete new record/rowtype to return.
-
-    Exceptions
-
-        PostgreSQL  doesn't  have  a  very  smart  exception handling
-        model. Whenever the  parser,  planner/optimizer  or  executor
-        decide  that  a statement cannot be processed any longer, the
-        whole transaction gets aborted and the the system jumps  back
-        into  the mainloop using longjmp() to get the next query from
-        the client application.
-
-        It is possible to hook into the longjmp() mechanism to notice
-        that this happens. But currently it's impossible to tell what
-        really  caused  the  abort  (input/output  conversion  error,
-        floating point error, parse error) And it's possible that the
-        backend  is  in  an  inconsistent  state  at  this  point  so
-        returning  to  the  upper  executor  or issuing more commands
-        might corrupt the whole database.
-
-        Thus,  the  only  thing  PL/pgSQL  currently  does  when   it
-        encounters an abort during execution of a function or trigger
-        procedure is to write  some  additional  DEBUG  log  messages
-        telling  in which function and where (line number and type of
-        statement) this happened.
-
-        This might change in the future.
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