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提交 69cd5efb 编写于 作者: B Bruce Momjian
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......@@ -445,3 +445,1253 @@ query cache in shared memory...etc. Too much queries, but less answers :-)
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Date: Fri, 12 Apr 2002 09:51:16 +0200
From: Karel Zak <zakkr@zf.jcu.cz>
To: pgsql-hackers@postgresql.org
cc: Hiroshi Inoue <Inoue@tpf.co.jp>
Subject: Re: [HACKERS] 7.3 schedule
Message-ID: <20020412095116.B6370@zf.jcu.cz>
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In-Reply-To: <20020412004134.5d35a2dd.nconway@klamath.dyndns.org>; from nconway@klamath.dyndns.org on Fri, Apr 12, 2002 at 12:41:34AM -0400
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On Fri, Apr 12, 2002 at 12:41:34AM -0400, Neil Conway wrote:
> On Fri, 12 Apr 2002 12:58:01 +0900
> "Hiroshi Inoue" <Inoue@tpf.co.jp> wrote:
> >
> > Just a confirmation.
> > Someone is working on PREPARE/EXECUTE ?
> > What about Karel's work ?
Right question :-)
> I am. My work is based on Karel's stuff -- at the moment I'm still
> basically working on getting Karel's patch to play nicely with
> current sources; once that's done I'll be addressing whatever
> issues are stopping the code from getting into CVS.
My patch (qcache) for PostgreSQL 7.0 is available at
ftp://ftp2.zf.jcu.cz/users/zakkr/pg/.
I very look forward to Neil's work on this.
Notes:
* It's experimental patch, but usable. All features below mentioned
works.
* PREPARE/EXECUTE is not only SQL statements, I think good idea is
create something common and robus for query-plan caching,
beacuse there is for example SPI too. The RI triggers are based
on SPI_saveplan().
* My patch knows EXECUTE INTO feature:
PREPARE foo AS SELECT * FROM pg_class WHERE relname ~~ $1 USING text;
EXECUTE foo USING 'pg%'; <-- standard select
EXECUTE foo INTO TEMP newtab USING 'pg%'; <-- select into
* The patch allows store query-planns to shared memory and is
possible EXECUTE it at more backends (over same DB) and planns
are persistent across connetions. For this feature I create special
memory context subsystem (like current aset.c, but it works with
IPC shared memory).
This is maybe too complex solution and (maybe) sufficient is cache
query in one backend only. I know unbelief about this shared
memory solution (Tom?).
Karel
My experimental patch README (excuse my English):
Implementation
~~~~~~~~~~~~~~
The qCache allows save queryTree and queryPlan. There is available are
two space for data caching.
LOCAL - data are cached in backend non-shared memory and data aren't
available in other backends.
SHARE - data are cached in backend shared memory and data are
visible in all backends.
Because size of share memory pool is limited and it is set during
postmaster start up, the qCache must remove all old planns if pool is
full. You can mark each entry as "REMOVEABLE" or "NOTREMOVEABLE".
A removeable entry is removed if pool is full.
A not-removeable entry must be removed via qCache_Remove() or
the other routines. The qCache not remove this entry itself.
All records in qCache are cached (in the hash table) under some key.
The qCache knows two alternate of key --- "KEY_STRING" and "KEY_BINARY".
The qCache API not allows access to shared memory, all cached planns that
API returns are copy to CurrentMemoryContext. All (qCache_ ) routines lock
shmem itself (exception is qCache_RemoveOldest_ShareRemoveAble()).
- for locking is used spin lock.
Memory management
~~~~~~~~~~~~~~~~~
The qCache use for qCache's shared pool its memory context independent on
standard aset/mcxt, but use compatible API --- it allows to use standard
palloc() (it is very needful for basic plan-tree operations, an example
for copyObject()). The qCache memory management is very simular to current
aset.c code. It is chunk-ed blocks too, but the block is smaller - 1024b.
The number of blocks is available set in postmaster 'argv' via option
'-Z'.
For plan storing is used separate MemoryContext for each plan, it
is good idea (Hiroshi's ?), bucause create new context is simple and
inexpensive and allows easy destroy (free) cached plan. This method is
used in my SPI overhaul instead TopMemoryContext feeding.
Postmaster
~~~~~~~~~~
The query cache memory is init during potmaster startup. The size of
query cache pool is set via '-Z <number-of-blocks>' switch --- default
is 100 blocks where 1 block = 1024b, it is sufficient for 20-30 cached
planns. One query needs somewhere 3-10 blocks, for example query like
PREPARE sel AS SELECT * FROM pg_class;
needs 10Kb, because table pg_class has very much columns.
Note: for development I add SQL function: "SELECT qcache_state();",
this routine show usage of qCache.
SPI
~~~
I a little overwrite SPI save plan method and remove TopMemoryContext
"feeding".
Standard SPI:
SPI_saveplan() - save each plan to separate standard memory context.
SPI_freeplan() - free plan.
By key SPI:
It is SPI interface for query cache and allows save planns to SHARED
or LOCAL cache 'by' arbitrary key (string or binary). Routines:
SPI_saveplan_bykey() - save plan to query cache
SPI_freeplan_bykey() - remove plan from query cache
SPI_fetchplan_bykey() - fetch plan saved in query cache
SPI_execp_bykey() - execute (via SPI) plan saved in query
cache
- now, users can write functions that save planns to shared memory
and planns are visible in all backend and are persistent arcoss
connection.
Example:
~~~~~~~
/* ----------
* Save/exec query from shared cache via string key
* ----------
*/
int keySize = 0;
flag = SPI_BYKEY_SHARE | SPI_BYKEY_STRING;
char *key = "my unique key";
res = SPI_execp_bykey(values, nulls, tcount, key, flag, keySize);
if (res == SPI_ERROR_PLANNOTFOUND)
{
/* --- not plan in cache - must create it --- */
void *plan;
plan = SPI_prepare(querystr, valnum, valtypes);
SPI_saveplan_bykey(plan, key, keySize, flag);
res = SPI_execute(plan, values, Nulls, tcount);
}
elog(NOTICE, "Processed: %d", SPI_processed);
PREPARE/EXECUTE
~~~~~~~~~~~~~~~
* Syntax:
PREPARE <name> AS <query>
[ USING type, ... typeN ]
[ NOSHARE | SHARE | GLOBAL ]
EXECUTE <name>
[ INTO [ TEMPORARY | TEMP ] [ TABLE ] new_table ]
[ USING val, ... valN ]
[ NOSHARE | SHARE | GLOBAL ]
DEALLOCATE PREPARE
[ <name> [ NOSHARE | SHARE | GLOBAL ]]
[ ALL | ALL INTERNAL ]
I know that it is a little out of SQL92... (use CREATE/DROP PLAN instead
this?) --- what mean SQL standard guru?
* Where:
NOSHARE --- cached in local backend query cache - not accessable
from the others backends and not is persisten a across
conection.
SHARE --- cached in shared query cache and accessable from
all backends which work over same database.
GLOBAL --- cached in shared query cache and accessable from
all backends and all databases.
- default is 'SHARE'
Deallocate:
ALL --- deallocate all users's plans
ALL INTERNAL --- deallocate all internal plans, like planns
cached via SPI. It is needful if user
alter/drop table ...etc.
* Parameters:
"USING" part in the prepare statement is for datetype setting for
paremeters in the query. For example:
PREPARE sel AS SELECT * FROM pg_class WHERE relname ~~ $1 USING text;
EXECUTE sel USING 'pg%';
* Limitation:
- prepare/execute allow use full statement of SELECT/INSERT/DELETE/
UPDATE.
- possible is use union, subselects, limit, ofset, select-into
Performance:
~~~~~~~~~~~
* the SPI
- I for my tests a little change RI triggers to use SPI by_key API
and save planns to shared qCache instead to internal RI hash table.
The RI use very simple (for parsing) queries and qCache interest is
not visible. It's better if backend very often startup and RI check
always same tables. In this situation speed go up --- 10-12%.
(This snapshot not include this RI change.)
But all depend on how much complicate for parser is query in
trigger.
* PREPARE/EXECUTE
- For tests I use query that not use some table (the executor is
in boredom state), but is difficult for the parser. An example:
SELECT 'a text ' || (10*10+(100^2))::text || ' next text ' || cast
(date_part('year', timestamp 'now') AS text );
- (10000 * this query):
standard select: 54 sec
via prepare/execute: 4 sec (93% better)
IMHO it is nod bad.
- For standard query like:
SELECT u.usename, r.relname FROM pg_class r, pg_user u WHERE
r.relowner = u.usesysid;
it is with PREPARE/EXECUTE 10-20% faster.
--
Karel Zak <zakkr@zf.jcu.cz>
http://home.zf.jcu.cz/~zakkr/
C, PostgreSQL, PHP, WWW, http://docs.linux.cz, http://mape.jcu.cz
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From pgsql-hackers-owner+M21228@postgresql.org Fri Apr 12 10:15:34 2002
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To: Karel Zak <zakkr@zf.jcu.cz>
cc: pgsql-hackers@postgresql.org, Neil Conway <nconway@klamath.dyndns.org>
Subject: Re: [HACKERS] 7.3 schedule
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References: <GNELIHDDFBOCMGBFGEFOGEBHCCAA.chriskl@familyhealth.com.au> <3CB52C54.4020507@freaky-namuh.com> <20020411115434.201ff92f.nconway@klamath.dyndns.org> <3CB61DAB.5010601@freaky-namuh.com> <24184.1018581907@sss.pgh.pa.us> <3CB65B49.93F2F790@tpf.co.jp> <20020412004134.5d35a2dd.nconway@klamath.dyndns.org> <20020412095116.B6370@zf.jcu.cz>
Comments: In-reply-to Karel Zak <zakkr@zf.jcu.cz>
message dated "Fri, 12 Apr 2002 09:51:16 +0200"
Date: Fri, 12 Apr 2002 10:14:26 -0400
Message-ID: <27235.1018620866@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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Karel Zak <zakkr@zf.jcu.cz> writes:
> * The patch allows store query-planns to shared memory and is
> possible EXECUTE it at more backends (over same DB) and planns
> are persistent across connetions. For this feature I create special
> memory context subsystem (like current aset.c, but it works with
> IPC shared memory).
> This is maybe too complex solution and (maybe) sufficient is cache
> query in one backend only. I know unbelief about this shared
> memory solution (Tom?).
Yes, that is the part that was my sticking point last time around.
(1) Because shared memory cannot be extended on-the-fly, I think it is
a very bad idea to put data structures in there without some well
thought out way of predicting/limiting their size. (2) How the heck do
you get rid of obsoleted cached plans, if the things stick around in
shared memory even after you start a new backend? (3) A shared cache
requires locking; contention among multiple backends to access that
shared resource could negate whatever performance benefit you might hope
to realize from it.
A per-backend cache kept in local memory avoids all of these problems,
and I have seen no numbers to make me think that a shared plan cache
would achieve significantly more performance benefit than a local one.
regards, tom lane
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From pgsql-hackers-owner+M21233@postgresql.org Fri Apr 12 12:26:32 2002
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200204121621.g3CGL4310492@candle.pha.pa.us>
Subject: Re: [HACKERS] 7.3 schedule
In-Reply-To: <27235.1018620866@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 12 Apr 2002 12:21:04 -0400 (EDT)
cc: Karel Zak <zakkr@zf.jcu.cz>, pgsql-hackers@postgresql.org,
Neil Conway <nconway@klamath.dyndns.org>
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Tom Lane wrote:
> Karel Zak <zakkr@zf.jcu.cz> writes:
> > * The patch allows store query-planns to shared memory and is
> > possible EXECUTE it at more backends (over same DB) and planns
> > are persistent across connetions. For this feature I create special
> > memory context subsystem (like current aset.c, but it works with
> > IPC shared memory).
> > This is maybe too complex solution and (maybe) sufficient is cache
> > query in one backend only. I know unbelief about this shared
> > memory solution (Tom?).
>
> Yes, that is the part that was my sticking point last time around.
> (1) Because shared memory cannot be extended on-the-fly, I think it is
> a very bad idea to put data structures in there without some well
> thought out way of predicting/limiting their size. (2) How the heck do
> you get rid of obsoleted cached plans, if the things stick around in
> shared memory even after you start a new backend? (3) A shared cache
> requires locking; contention among multiple backends to access that
> shared resource could negate whatever performance benefit you might hope
> to realize from it.
>
> A per-backend cache kept in local memory avoids all of these problems,
> and I have seen no numbers to make me think that a shared plan cache
> would achieve significantly more performance benefit than a local one.
Certainly a shared cache would be good for apps that connect to issue a
single query frequently. In such cases, there would be no local cache
to use.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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From pgsql-hackers-owner+M21234@postgresql.org Fri Apr 12 12:44:12 2002
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Subject: Re: [HACKERS] 7.3 schedule
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Tom Lane wrote:
> Yes, that is the part that was my sticking point last time around.
> (1) Because shared memory cannot be extended on-the-fly, I think it is
> a very bad idea to put data structures in there without some well
> thought out way of predicting/limiting their size. (2) How the heck do
> you get rid of obsoleted cached plans, if the things stick around in
> shared memory even after you start a new backend? (3) A shared cache
> requires locking; contention among multiple backends to access that
> shared resource could negate whatever performance benefit you might hope
> to realize from it.
>
> A per-backend cache kept in local memory avoids all of these problems,
> and I have seen no numbers to make me think that a shared plan cache
> would achieve significantly more performance benefit than a local one.
>
Oracle's implementation is a shared cache for all plans. This was
introduced in Oracle 6 or 7 (I don't remember which anymore). The net
effect was that in general there was a significant performance
improvement with the shared cache. However poorly written apps can now
bring the Oracle database to its knees because of the locking issues
associated with the shared cache. For example if the most frequently
run sql statements are coded poorly (i.e. they don't use bind variables,
eg. 'select bar from foo where foobar = $1' vs. 'select bar from foo
where foobar = || somevalue' (where somevalue is likely to be
different on every call)) the shared cache doesn't help and its overhead
becomes significant.
thanks,
--Barry
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From pgsql-hackers-owner+M21237@postgresql.org Fri Apr 12 12:50:28 2002
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200204121649.g3CGnbw12950@candle.pha.pa.us>
Subject: Re: [HACKERS] 7.3 schedule
In-Reply-To: <3CB70E7C.3090801@xythos.com>
To: Barry Lind <barry@xythos.com>
Date: Fri, 12 Apr 2002 12:49:37 -0400 (EDT)
cc: Tom Lane <tgl@sss.pgh.pa.us>, Karel Zak <zakkr@zf.jcu.cz>,
pgsql-hackers@postgresql.org, Neil Conway <nconway@klamath.dyndns.org>
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Barry Lind wrote:
> Oracle's implementation is a shared cache for all plans. This was
> introduced in Oracle 6 or 7 (I don't remember which anymore). The net
> effect was that in general there was a significant performance
> improvement with the shared cache. However poorly written apps can now
> bring the Oracle database to its knees because of the locking issues
> associated with the shared cache. For example if the most frequently
> run sql statements are coded poorly (i.e. they don't use bind variables,
> eg. 'select bar from foo where foobar = $1' vs. 'select bar from foo
> where foobar = || somevalue' (where somevalue is likely to be
> different on every call)) the shared cache doesn't help and its overhead
> becomes significant.
This is very interesting. We have always been concerned that shared
cache invalidation could cause more of a performance problem that the
shared cache gives benefit, and it sounds like you are saying exactly
that.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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From pgsql-hackers-owner+M21238@postgresql.org Fri Apr 12 12:51:55 2002
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To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Karel Zak <zakkr@zf.jcu.cz>, pgsql-hackers@postgresql.org,
Neil Conway <nconway@klamath.dyndns.org>
Subject: Re: [HACKERS] 7.3 schedule
In-Reply-To: <200204121621.g3CGL4310492@candle.pha.pa.us>
References: <200204121621.g3CGL4310492@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
message dated "Fri, 12 Apr 2002 12:21:04 -0400"
Date: Fri, 12 Apr 2002 12:51:26 -0400
Message-ID: <27964.1018630286@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Certainly a shared cache would be good for apps that connect to issue a
> single query frequently. In such cases, there would be no local cache
> to use.
We have enough other problems with the single-query-per-connection
scenario that I see no reason to believe that a shared plan cache will
help materially. The correct answer for those folks will *always* be
to find a way to reuse the connection.
regards, tom lane
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From pgsql-hackers-owner+M21241@postgresql.org Fri Apr 12 16:25:46 2002
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Date: Fri, 12 Apr 2002 16:24:48 -0400
From: Neil Conway <nconway@klamath.dyndns.org>
To: "Bruce Momjian" <pgman@candle.pha.pa.us>
cc: tgl@sss.pgh.pa.us, zakkr@zf.jcu.cz, pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] 7.3 schedule
Message-ID: <20020412162448.4d46d747.nconway@klamath.dyndns.org>
In-Reply-To: <200204121621.g3CGL4310492@candle.pha.pa.us>
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<200204121621.g3CGL4310492@candle.pha.pa.us>
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On Fri, 12 Apr 2002 12:21:04 -0400 (EDT)
"Bruce Momjian" <pgman@candle.pha.pa.us> wrote:
> Tom Lane wrote:
> > A per-backend cache kept in local memory avoids all of these problems,
> > and I have seen no numbers to make me think that a shared plan cache
> > would achieve significantly more performance benefit than a local one.
>
> Certainly a shared cache would be good for apps that connect to issue a
> single query frequently. In such cases, there would be no local cache
> to use.
One problem with this kind of scenario is: what to do if the plan no
longer exists for some reason? (e.g. the code that was supposed to be
PREPARE-ing your statements failed to execute properly, or the cached
plan has been evicted from shared memory, or the database was restarted,
etc.) -- EXECUTE in and of itself won't have enough information to do
anything useful. We could perhaps provide a means for an application
to test for the existence of a cached plan (in which case the
application developer will need to add logic to their application
to re-prepare the query if necessary, which could get complicated).
Cheers,
Neil
--
Neil Conway <neilconway@rogers.com>
PGP Key ID: DB3C29FC
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From pgsql-hackers-owner+M21242@postgresql.org Fri Apr 12 17:27:24 2002
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200204122125.g3CLPVa14231@candle.pha.pa.us>
Subject: Re: [HACKERS] 7.3 schedule
In-Reply-To: <20020412162448.4d46d747.nconway@klamath.dyndns.org>
To: Neil Conway <nconway@klamath.dyndns.org>
Date: Fri, 12 Apr 2002 17:25:31 -0400 (EDT)
cc: tgl@sss.pgh.pa.us, zakkr@zf.jcu.cz, pgsql-hackers@postgresql.org
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Neil Conway wrote:
> On Fri, 12 Apr 2002 12:21:04 -0400 (EDT)
> "Bruce Momjian" <pgman@candle.pha.pa.us> wrote:
> > Tom Lane wrote:
> > > A per-backend cache kept in local memory avoids all of these problems,
> > > and I have seen no numbers to make me think that a shared plan cache
> > > would achieve significantly more performance benefit than a local one.
> >
> > Certainly a shared cache would be good for apps that connect to issue a
> > single query frequently. In such cases, there would be no local cache
> > to use.
>
> One problem with this kind of scenario is: what to do if the plan no
> longer exists for some reason? (e.g. the code that was supposed to be
> PREPARE-ing your statements failed to execute properly, or the cached
> plan has been evicted from shared memory, or the database was restarted,
> etc.) -- EXECUTE in and of itself won't have enough information to do
> anything useful. We could perhaps provide a means for an application
> to test for the existence of a cached plan (in which case the
> application developer will need to add logic to their application
> to re-prepare the query if necessary, which could get complicated).
Oh, are you thinking that one backend would do the PREPARE and another
one the EXECUTE? I can't see that working at all. I thought there
would some way to quickly test if the submitted query was in the cache,
but maybe that is too much of a performance penalty to be worth it.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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From tgl@sss.pgh.pa.us Fri Apr 12 17:36:17 2002
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To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Neil Conway <nconway@klamath.dyndns.org>, zakkr@zf.jcu.cz,
pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] 7.3 schedule
In-Reply-To: <200204122125.g3CLPVa14231@candle.pha.pa.us>
References: <200204122125.g3CLPVa14231@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
message dated "Fri, 12 Apr 2002 17:25:31 -0400"
Date: Fri, 12 Apr 2002 17:36:16 -0400
Message-ID: <10810.1018647376@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ORr
Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Oh, are you thinking that one backend would do the PREPARE and another
> one the EXECUTE? I can't see that working at all.
Uh, why exactly were you advocating a shared cache then? Wouldn't that
be exactly the *point* of a shared cache?
regards, tom lane
From pgsql-hackers-owner+M21245@postgresql.org Fri Apr 12 17:39:13 2002
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200204122138.g3CLcFX16347@candle.pha.pa.us>
Subject: Re: [HACKERS] 7.3 schedule
In-Reply-To: <10810.1018647376@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 12 Apr 2002 17:38:15 -0400 (EDT)
cc: Neil Conway <nconway@klamath.dyndns.org>, zakkr@zf.jcu.cz,
pgsql-hackers@postgresql.org
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Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Oh, are you thinking that one backend would do the PREPARE and another
> > one the EXECUTE? I can't see that working at all.
>
> Uh, why exactly were you advocating a shared cache then? Wouldn't that
> be exactly the *point* of a shared cache?
I thought it would somehow compare the SQL query string to the cached
plans and if it matched, it would use that plan rather than make a new
one. Any DDL statement would flush the cache.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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From pgsql-hackers-owner+M21246@postgresql.org Fri Apr 12 17:56:58 2002
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Subject: Re: [HACKERS] 7.3 schedule
Date: Fri, 12 Apr 2002 14:59:15 -0700
Message-ID: <D90A5A6C612A39408103E6ECDD77B82906F42C@voyager.corporate.connx.com>
Thread-Topic: [HACKERS] 7.3 schedule
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From: "Dann Corbit" <DCorbit@connx.com>
To: "Bruce Momjian" <pgman@candle.pha.pa.us>, "Tom Lane" <tgl@sss.pgh.pa.us>
cc: "Neil Conway" <nconway@klamath.dyndns.org>, <zakkr@zf.jcu.cz>,
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-----Original Message-----
From: Bruce Momjian [mailto:pgman@candle.pha.pa.us]
Sent: Friday, April 12, 2002 2:38 PM
To: Tom Lane
Cc: Neil Conway; zakkr@zf.jcu.cz; pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] 7.3 schedule
Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Oh, are you thinking that one backend would do the PREPARE and
another
> > one the EXECUTE? I can't see that working at all.
>
> Uh, why exactly were you advocating a shared cache then? Wouldn't
that
> be exactly the *point* of a shared cache?
I thought it would somehow compare the SQL query string to the cached
plans and if it matched, it would use that plan rather than make a new
one. Any DDL statement would flush the cache.
>>-------------------------------------------------------------------
Many applications will have similar queries coming from lots of
different end-users. Imagine an order-entry program where people are
ordering parts. Many of the queries might look like this:
SELECT part_number FROM parts WHERE part_id = 12324 AND part_cost
< 12.95
In order to cache this query, we first parse it to replace the data
fields with paramter markers.
Then it looks like this:
SELECT part_number FROM parts WHERE part_id = ? AND part_cost < ?
{in the case of a 'LIKE' query or some other query where you can use
key information, you might have a symbolic replacement like this:
WHERE field LIKE '{D}%' to indicate that the key can be used}
Then, we make sure that the case is consistent by either capitalizing
the whole query or changing it all into lower case:
select part_number from parts where part_id = ? and part_cost < ?
Then, we run a checksum on the parameterized string.
The checksum might be used as a hash table key, where we keep some
additional information like how stale the entry is, and a pointer to
the actual parameterized SQL (in case the hash key has a collision
it would be simply wrong to run an incorrect query for obvious enough
reasons).
Now, if there are a huge number of users of the same application, it
makes sense that the probabilities of reusing queries goes up with
the number of users of the same application. Therefore, I would
advocate that the cache be kept in shared memory.
Consider a single application with 100 different queries. Now, add
one user, ten users, 100 users, ... 10,000 users and you can see
that the benefit would be greater and greater as we add users.
<<-------------------------------------------------------------------
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From pgsql-hackers-owner+M21270@postgresql.org Sat Apr 13 02:30:47 2002
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From: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
To: "Barry Lind" <barry@xythos.com>, "Tom Lane" <tgl@sss.pgh.pa.us>
cc: "Karel Zak" <zakkr@zf.jcu.cz>, <pgsql-hackers@postgresql.org>,
"Neil Conway" <nconway@klamath.dyndns.org>
References: <GNELIHDDFBOCMGBFGEFOGEBHCCAA.chriskl@familyhealth.com.au> <3CB52C54.4020507@freaky-namuh.com> <20020411115434.201ff92f.nconway@klamath.dyndns.org> <3CB61DAB.5010601@freaky-namuh.com> <24184.1018581907@sss.pgh.pa.us> <3CB65B49.93F2F790@tpf.co.jp> <20020412004134.5d35a2dd.nconway@klamath.dyndns.org> <20020412095116.B6370@zf.jcu.cz> <27235.1018620866@sss.pgh.pa.us> <3CB70E7C.3090801@xythos.com>
Subject: Re: [HACKERS] 7.3 schedule
Date: Sat, 13 Apr 2002 14:21:50 +0800
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> > thought out way of predicting/limiting their size. (2) How the heck do
> > you get rid of obsoleted cached plans, if the things stick around in
> > shared memory even after you start a new backend? (3) A shared cache
> > requires locking; contention among multiple backends to access that
> > shared resource could negate whatever performance benefit you might hope
> > to realize from it.
I don't understand all these locking problems? Surely the only lock a
transaction would need on a stored query is one that prevents the cache
invalidation mechanism from deleting it out from under it? Surely this
means that there would be tonnes of readers on the cache - none of them
blocking each other, and the odd invalidation event that needs a complete
lock?
Also, as for invalidation, there probably could be just two reasons to
invalidate a query in the cache. (1) The cache is running out of space and
you use LRU or something to remove old queries, or (2) someone runs ANALYZE,
in which case all cached queries should just be flushed? If they specify an
actual table to analyze, then just drop all queries on the table.
Could this cache mechanism be used to make views fast as well? You could
cache the queries that back views on first use, and then they can follow the
above rules for flushing...
Chris
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From pgsql-hackers-owner+M21276@postgresql.org Sat Apr 13 11:48:51 2002
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To: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
cc: "Barry Lind" <barry@xythos.com>, "Karel Zak" <zakkr@zf.jcu.cz>,
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Subject: Re: [HACKERS] 7.3 schedule
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Comments: In-reply-to "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
message dated "Sat, 13 Apr 2002 14:21:50 +0800"
Date: Sat, 13 Apr 2002 11:46:01 -0400
Message-ID: <15740.1018712761@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
> thought out way of predicting/limiting their size. (2) How the heck do
> you get rid of obsoleted cached plans, if the things stick around in
> shared memory even after you start a new backend? (3) A shared cache
> requires locking; contention among multiple backends to access that
> shared resource could negate whatever performance benefit you might hope
> to realize from it.
> I don't understand all these locking problems?
Searching the cache and inserting/deleting entries in the cache probably
have to be mutually exclusive; concurrent insertions probably won't work
either (at least not without a remarkably intelligent data structure).
Unless the cache hit rate is remarkably high, there are going to be lots
of insertions --- and, at steady state, an equal rate of deletions ---
leading to lots of contention.
This could possibly be avoided if the cache is not used for all query
plans but only for explicitly PREPAREd plans, so that only explicit
EXECUTEs would need to search it. But that approach also makes a
sizable dent in the usefulness of the cache to begin with.
regards, tom lane
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From pgsql-hackers-owner+M21280@postgresql.org Sat Apr 13 14:36:34 2002
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Date: Sat, 13 Apr 2002 14:35:39 -0400
From: Neil Conway <nconway@klamath.dyndns.org>
To: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
cc: barry@xythos.com, tgl@sss.pgh.pa.us, zakkr@zf.jcu.cz,
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Subject: Re: [HACKERS] 7.3 schedule
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On Sat, 13 Apr 2002 14:21:50 +0800
"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> wrote:
> Could this cache mechanism be used to make views fast as well?
The current PREPARE/EXECUTE code will speed up queries that use
rules of any kind, including views: the query plan is cached after
it has been rewritten as necessary, so (AFAIK) this should mean
that rules will be evaluated once when the query is PREPAREd, and
then cached for subsequent EXECUTE commands.
Cheers,
Neil
--
Neil Conway <neilconway@rogers.com>
PGP Key ID: DB3C29FC
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From pgsql-hackers-owner+M21309@postgresql.org Sun Apr 14 15:22:44 2002
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Date: Sun, 14 Apr 2002 21:21:44 +0200
From: Karel Zak <zakkr@zf.jcu.cz>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: Bruce Momjian <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org,
Neil Conway <nconway@klamath.dyndns.org>
Subject: Re: [HACKERS] 7.3 schedule
Message-ID: <20020414212144.A12196@zf.jcu.cz>
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In-Reply-To: <27964.1018630286@sss.pgh.pa.us>; from tgl@sss.pgh.pa.us on Fri, Apr 12, 2002 at 12:51:26PM -0400
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On Fri, Apr 12, 2002 at 12:51:26PM -0400, Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Certainly a shared cache would be good for apps that connect to issue a
> > single query frequently. In such cases, there would be no local cache
> > to use.
>
> We have enough other problems with the single-query-per-connection
> scenario that I see no reason to believe that a shared plan cache will
> help materially. The correct answer for those folks will *always* be
> to find a way to reuse the connection.
My query cache was write for 7.0. If some next release will use
pre-forked backend and after a client disconnection the backend will
still alives and waits for new client the shared cache is (maybe:-) not
needful. The current backend fork model is killer of all possible
caching.
We have more caches. I hope persistent backend help will help to all
and I'm sure that speed will grow up with persistent backend and
persistent caches without shared memory usage. There I can agree with
Tom :-)
Karel
--
Karel Zak <zakkr@zf.jcu.cz>
http://home.zf.jcu.cz/~zakkr/
C, PostgreSQL, PHP, WWW, http://docs.linux.cz, http://mape.jcu.cz
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From pgsql-hackers-owner+M21321@postgresql.org Sun Apr 14 20:40:08 2002
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Date: Sun, 14 Apr 2002 20:38:48 -0400 (EDT)
From: Brian Bruns <camber@ais.org>
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To: Hannu Krosing <hannu@tm.ee>
cc: <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] 7.3 schedule
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On 13 Apr 2002, Hannu Krosing wrote:
> On Fri, 2002-04-12 at 03:04, Brian Bruns wrote:
> > On 11 Apr 2002, Hannu Krosing wrote:
> >
> > > IIRC someone started work on modularising the network-related parts with
> > > a goal of supporting DRDA (DB2 protocol) and others in future.
> >
> > That was me, although I've been bogged down lately, and haven't been able
> > to get back to it.
>
> Has any of your modularisation work got into CVS yet ?
No, Bruce didn't like the way I did certain things, and had some qualms
about the value of supporting multiple wire protocols IIRC. Plus the
patch was not really ready for primetime yet.
I'm hoping to get back to it soon and sync it with the latest CVS, and
clean up the odds and ends.
> > DRDA, btw, is not just a DB2 protocol but an opengroup
> > spec that hopefully will someday be *the* standard on the wire database
> > protocol. DRDA handles prepare/execute and is completely binary in
> > representation, among other advantages.
>
> What about extensibility - is there some predefined way of adding new
> types ?
Not really, there is some ongoing standards activity adding some new
features. The list of supported types is pretty impressive, anything in
particular you are looking for?
> Also, does it handle NOTIFY ?
I don't know the answer to this. The spec is pretty huge, so it may, but
I haven't seen it.
Even if it is supported as a secondary protocol, I believe there is alot
of value in having a single database protocol standard. (why else would I
be doing it!). I'm also looking into what it will take to do the same for
MySQL and Firebird. Hopefully they will be receptive to the idea as well.
> ----------------
> Hannu
Cheers,
Brian
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