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Re: Additional current timestamp values

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Neil Conway <neilc(at)samurai(dot)com>, Brendan Jurd <direvus(at)gmail(dot)com>, pgsql-patches(at)postgresql(dot)org
Subject: Re: Additional current timestamp values
Date: 2006-04-23 18:30:51
Message-ID: 200604231830.k3NIUpE22328@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
I am not happy with my patch and am going to try a more comprehensive
restructuring --- will post later.

---------------------------------------------------------------------------

Bruce Momjian wrote:
> Tom Lane wrote:
> > Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > > Tom Lane wrote:
> > >> The patch as given strikes me as pretty broken --- it does not advance
> > >> statement_timestamp when I would expect (AFAICS it only sets it during
> > >> transaction start).
> > 
> > > Uh, it does advance:
> > 
> > But not once per statement --- in reality, you get a fairly arbitrary
> > behavior that will advance in some cases and not others when dealing
> > with a multi-statement querystring.  Your example showing that it fails
> > to advance in a psql -c string shows this ... don't you think most
> > people would call that a bug?
> > 
> > If it's "statement" timestamp then I think it ought to advance once per
> > SQL statement, which this isn't doing.  (As I already said, though, that
> > isn't the behavior I really want.  My point is just that the code's
> > behavior is an extremely strange, nonintuitive definition of the word
> > "statement".)
> > 
> > > I have always been confused if
> > > statement_timeout times queries inside server-side functions, for
> > > example.  I don't think it should.
> > 
> > That's exactly my point; I agree that we don't want it doing that,
> > but that being the case, "statement" isn't a great name for the units
> > that we are actually processing.  We're really wanting to do these
> > things once per client command, or maybe per client query would be a
> > better name.
> 
> I have updated my patch based on community comments.  One cleanup is
> that I now set statement_timestamp(), and then base
> transaction_timestamp() (aka now()) on the statement_timestamp of BEGIN,
> which is a much cleaner API.
> 
> As far as how often statement_timestamp() is called, when a "Q" query
> arrives, it calls exec_simple_query(), which calls start_xact_command()
> before it parses anything, setting the transaction start.  It is called
> inside the per-command loop, but it does nothing unless
> finish_xact_command() was called to finish a transaction.
> 
> (Is there some double-processing here for BEGIN because it will re-run
> the initialization stuff?) 
> 
> I also documented how statement_timestamp behaves when multiple
> statements are in the same query string, and when called from functions.
> 
> One side-affect of tracking transaction_timestamp based on
> statement_timestamp() is if multiple statements are sent in a single
> query string, and multiple transactions are used, statement_timestamp
> will be advanced so transaction_timestamp() can vary.  Again, not ideal,
> but probably the cleanest we are going to be able to do.  If we decided
> to just have statement_timestamp be the arrival of the string always, we
> are going to incur additional gettimeofday() calls and the code is going
> to be more complex.
> 
> FYI, this is exactly how statement_timeout behaves, and no one has
> complained about it.
> 
> The only other approach would be to put the statement_timestamp()
> setting call in exec_simple_query(), and in all the protocol-level
> functions, and fastpath.  You then also need to do a separate call for
> transaction_timestamp() because you want that to advance if multiple
> transactions are in the same query string.
> 
> If we want to take that approach, should statement_timeout code also be
> moved around?
> 
> See my other post about the use of the term "statement".  I don't think
> most people think about sending multiple statements, so if we document
> its behavior, that is good enough.
> 
> -- 
>   Bruce Momjian   http://candle.pha.pa.us
>   EnterpriseDB    http://www.enterprisedb.com
> 
>   + If your life is a hard drive, Christ can be your backup. +

> Index: doc/src/sgml/func.sgml
> ===================================================================
> RCS file: /cvsroot/pgsql/doc/src/sgml/func.sgml,v
> retrieving revision 1.313
> diff -c -c -r1.313 func.sgml
> *** doc/src/sgml/func.sgml	10 Mar 2006 20:15:25 -0000	1.313
> --- doc/src/sgml/func.sgml	23 Apr 2006 02:26:19 -0000
> ***************
> *** 5303,5308 ****
> --- 5303,5317 ----
>       <primary>now</primary>
>      </indexterm>
>      <indexterm>
> +     <primary>transaction_timestamp</primary>
> +    </indexterm>
> +    <indexterm>
> +     <primary>statement_timestamp</primary>
> +    </indexterm>
> +    <indexterm>
> +     <primary>clock_timestamp</primary>
> +    </indexterm>
> +    <indexterm>
>       <primary>timeofday</primary>
>      </indexterm>
>   
> ***************
> *** 5358,5364 ****
>          <row>
>           <entry><literal><function>current_timestamp</function></literal></entry>
>           <entry><type>timestamp with time zone</type></entry>
> !         <entry>Date and time; see <xref linkend="functions-datetime-current">
>           </entry>
>           <entry></entry>
>           <entry></entry>
> --- 5367,5373 ----
>          <row>
>           <entry><literal><function>current_timestamp</function></literal></entry>
>           <entry><type>timestamp with time zone</type></entry>
> !         <entry>Date and time of start of current transaction; see <xref linkend="functions-datetime-current">
>           </entry>
>           <entry></entry>
>           <entry></entry>
> ***************
> *** 5474,5481 ****
>          <row>
>           <entry><literal><function>now</function>()</literal></entry>
>           <entry><type>timestamp with time zone</type></entry>
> !         <entry>Current date and time (equivalent to
> !          <function>current_timestamp</function>); see <xref linkend="functions-datetime-current">
>           </entry>
>           <entry></entry>
>           <entry></entry>
> --- 5483,5518 ----
>          <row>
>           <entry><literal><function>now</function>()</literal></entry>
>           <entry><type>timestamp with time zone</type></entry>
> !         <entry>Date and time of start of current transaction (equivalent to
> !          <function>CURRENT_TIMESTAMP</function>); see <xref linkend="functions-datetime-current">
> !         </entry>
> !         <entry></entry>
> !         <entry></entry>
> !        </row>
> ! 
> !        <row>
> !         <entry><literal><function>transaction_timestamp</function>()</literal></entry>
> !         <entry><type>timestamp with time zone</type></entry>
> !         <entry>Date and time of start of current transaction (equivalent to
> !          <function>CURRENT_TIMESTAMP</function>); see <xref linkend="functions-datetime-current">
> !         </entry>
> !         <entry></entry>
> !         <entry></entry>
> !        </row>
> ! 
> !        <row>
> !         <entry><literal><function>statement_timestamp</function>()</literal></entry>
> !         <entry><type>timestamp with time zone</type></entry>
> !         <entry>Date and time of start of current statement; see <xref linkend="functions-datetime-current">
> !         </entry>
> !         <entry></entry>
> !         <entry></entry>
> !        </row>
> ! 
> !        <row>
> !         <entry><literal><function>clock_timestamp</function>()</literal></entry>
> !         <entry><type>timestamp with time zone</type></entry>
> !         <entry>Current date and time (changes during statement execution); see <xref linkend="functions-datetime-current">
>           </entry>
>           <entry></entry>
>           <entry></entry>
> ***************
> *** 5484,5490 ****
>          <row>
>           <entry><literal><function>timeofday</function>()</literal></entry>
>           <entry><type>text</type></entry>
> !         <entry>Current date and time; see <xref linkend="functions-datetime-current">
>           </entry>
>           <entry></entry>
>           <entry></entry>
> --- 5521,5528 ----
>          <row>
>           <entry><literal><function>timeofday</function>()</literal></entry>
>           <entry><type>text</type></entry>
> !         <entry>Current date and time (like <function>clock_timestamp</>), but as a Unix-style <type>text</> value; 
> !         see <xref linkend="functions-datetime-current">
>           </entry>
>           <entry></entry>
>           <entry></entry>
> ***************
> *** 6072,6078 ****
>     </sect2>
>   
>     <sect2 id="functions-datetime-current">
> !    <title>Current Date/Time</title>
>   
>      <indexterm>
>       <primary>date</primary>
> --- 6110,6116 ----
>     </sect2>
>   
>     <sect2 id="functions-datetime-current">
> !    <title>Date/Time of Transaction Start</title>
>   
>      <indexterm>
>       <primary>date</primary>
> ***************
> *** 6085,6092 ****
>      </indexterm>
>   
>      <para>
> !     The following functions are available to obtain the current date and/or
> !     time:
>   <synopsis>
>   CURRENT_DATE
>   CURRENT_TIME
> --- 6123,6130 ----
>      </indexterm>
>   
>      <para>
> !     The following functions are available to obtain the date and/or
> !     time of the start of the current transaction:
>   <synopsis>
>   CURRENT_DATE
>   CURRENT_TIME
> ***************
> *** 6147,6158 ****
>      </para>
>   
>      <para>
> -     The function <function>now()</function> is the traditional
> -     <productname>PostgreSQL</productname> equivalent to
> -     <function>CURRENT_TIMESTAMP</function>.
> -    </para>
> - 
> -    <para>
>       It is important to know that
>       <function>CURRENT_TIMESTAMP</function> and related functions return
>       the start time of the current transaction; their values do not
> --- 6185,6190 ----
> ***************
> *** 6160,6185 ****
>       the intent is to allow a single transaction to have a consistent
>       notion of the <quote>current</quote> time, so that multiple
>       modifications within the same transaction bear the same
> !     time stamp.
>      </para>
>   
> !    <note>
> !     <para>
> !      Other database systems may advance these values more
> !      frequently.
> !     </para>
> !    </note>
>   
>      <para>
> !     There is also the function <function>timeofday()</function> which
> !     returns the wall-clock time and advances during transactions.  For
> !     historical reasons <function>timeofday()</function> returns a
> !     <type>text</type> string rather than a <type>timestamp</type>
> !     value:
> ! <screen>
> ! SELECT timeofday();
> ! <lineannotation>Result: </lineannotation><computeroutput>Sat Feb 17 19:07:32.000126 2001 EST</computeroutput>
> ! </screen>
>      </para>
>   
>      <para>
> --- 6192,6222 ----
>       the intent is to allow a single transaction to have a consistent
>       notion of the <quote>current</quote> time, so that multiple
>       modifications within the same transaction bear the same
> !     time stamp.  Consider using <function>statement_timestamp</> or
> !     <function>clock_timestamp</> if you need something that changes
> !     more frequently.
>      </para>
>   
> !    <para>
> !     <function>CURRENT_TIMESTAMP</> might not be the 
> !     transaction start time on other database systems.
> !     For this reason, and for completeness, 
> !     <function>transaction_timestamp</> is provided.
> !     The function <function>now()</function> is the traditional
> !     <productname>PostgreSQL</productname> equivalent to
> !     the SQL-standard <function>CURRENT_TIMESTAMP</function>.
> !    </para>
>   
>      <para>
> !     <function>STATEMENT_TIMESTAMP</> is the time the statement
> !     arrived at the server from the client.  It is not the time
> !     the command started execution.  If multiple commands were
> !     sent as a single query string to the server, each command
> !     has the same <function>STATEMENT_TIMESTAMP</> because they
> !     all arrived at the same time.  Also, commands executed
> !     by server-side functions have a <function>STATEMENT_TIMESTAMP</>
> !     based on the time the client sent the query that triggered
> !     the function, not the time the function was executed.
>      </para>
>   
>      <para>
> Index: src/backend/access/transam/xact.c
> ===================================================================
> RCS file: /cvsroot/pgsql/src/backend/access/transam/xact.c,v
> retrieving revision 1.219
> diff -c -c -r1.219 xact.c
> *** src/backend/access/transam/xact.c	29 Mar 2006 21:17:37 -0000	1.219
> --- src/backend/access/transam/xact.c	23 Apr 2006 02:26:21 -0000
> ***************
> *** 172,177 ****
> --- 172,178 ----
>    * keep it inside the TransactionState stack.
>    */
>   static TimestampTz xactStartTimestamp;
> + static TimestampTz stmtStartTimestamp;
>   
>   /*
>    * GID to be used for preparing the current transaction.  This is also
> ***************
> *** 428,433 ****
> --- 429,452 ----
>   }
>   
>   /*
> +  *	GetCurrentStatementStartTimestamp
> +  */
> + TimestampTz
> + GetCurrentStatementStartTimestamp(void)
> + {
> + 	return stmtStartTimestamp;
> + }
> + 
> + /*
> +  *	SetCurrentStatementStartTimestamp
> +  */
> + void
> + SetCurrentStatementStartTimestamp(void)
> + {
> + 	stmtStartTimestamp = GetCurrentTimestamp();
> + }
> + 
> + /*
>    *	GetCurrentTransactionNestLevel
>    *
>    * Note: this will return zero when not inside any transaction, one when
> ***************
> *** 1367,1375 ****
>   	XactLockTableInsert(s->transactionId);
>   
>   	/*
> ! 	 * set now()
>   	 */
> ! 	xactStartTimestamp = GetCurrentTimestamp();
>   
>   	/*
>   	 * initialize current transaction state fields
> --- 1386,1394 ----
>   	XactLockTableInsert(s->transactionId);
>   
>   	/*
> ! 	 * now() and statement_timestamp() should be the same time
>   	 */
> ! 	xactStartTimestamp = stmtStartTimestamp;
>   
>   	/*
>   	 * initialize current transaction state fields
> Index: src/backend/tcop/postgres.c
> ===================================================================
> RCS file: /cvsroot/pgsql/src/backend/tcop/postgres.c,v
> retrieving revision 1.485
> diff -c -c -r1.485 postgres.c
> *** src/backend/tcop/postgres.c	22 Apr 2006 01:26:00 -0000	1.485
> --- src/backend/tcop/postgres.c	23 Apr 2006 02:26:24 -0000
> ***************
> *** 2000,2006 ****
>   
>   
>   /*
> !  * Convenience routines for starting/committing a single command.
>    */
>   static void
>   start_xact_command(void)
> --- 2000,2008 ----
>   
>   
>   /*
> !  *	Check if the newly-arrived query string needs to have an implicit
> !  *	transaction started.  Also set statement_timestamp() and optionally
> !  *	statement_timeout.
>    */
>   static void
>   start_xact_command(void)
> ***************
> *** 2009,2014 ****
> --- 2011,2018 ----
>   	{
>   		ereport(DEBUG3,
>   				(errmsg_internal("StartTransactionCommand")));
> + 
> + 		SetCurrentStatementStartTimestamp();
>   		StartTransactionCommand();
>   
>   		/* Set statement timeout running, if any */
> Index: src/backend/utils/adt/timestamp.c
> ===================================================================
> RCS file: /cvsroot/pgsql/src/backend/utils/adt/timestamp.c,v
> retrieving revision 1.162
> diff -c -c -r1.162 timestamp.c
> *** src/backend/utils/adt/timestamp.c	6 Mar 2006 22:49:16 -0000	1.162
> --- src/backend/utils/adt/timestamp.c	23 Apr 2006 02:26:26 -0000
> ***************
> *** 920,925 ****
> --- 920,937 ----
>   }
>   
>   Datum
> + statement_timestamp(PG_FUNCTION_ARGS)
> + {
> + 	PG_RETURN_TIMESTAMPTZ(GetCurrentStatementStartTimestamp());
> + }
> + 
> + Datum
> + clock_timestamp(PG_FUNCTION_ARGS)
> + {
> + 	PG_RETURN_TIMESTAMPTZ(GetCurrentTimestamp());
> + }
> + 
> + Datum
>   pgsql_postmaster_start_time(PG_FUNCTION_ARGS)
>   {
>   	PG_RETURN_TIMESTAMPTZ(PgStartTime);
> Index: src/include/access/xact.h
> ===================================================================
> RCS file: /cvsroot/pgsql/src/include/access/xact.h,v
> retrieving revision 1.81
> diff -c -c -r1.81 xact.h
> *** src/include/access/xact.h	24 Mar 2006 04:32:13 -0000	1.81
> --- src/include/access/xact.h	23 Apr 2006 02:26:27 -0000
> ***************
> *** 141,146 ****
> --- 141,148 ----
>   extern SubTransactionId GetCurrentSubTransactionId(void);
>   extern CommandId GetCurrentCommandId(void);
>   extern TimestampTz GetCurrentTransactionStartTimestamp(void);
> + extern TimestampTz GetCurrentStatementStartTimestamp(void);
> + extern void SetCurrentStatementStartTimestamp(void);
>   extern int	GetCurrentTransactionNestLevel(void);
>   extern bool TransactionIdIsCurrentTransactionId(TransactionId xid);
>   extern void CommandCounterIncrement(void);
> Index: src/include/catalog/pg_proc.h
> ===================================================================
> RCS file: /cvsroot/pgsql/src/include/catalog/pg_proc.h,v
> retrieving revision 1.405
> diff -c -c -r1.405 pg_proc.h
> *** src/include/catalog/pg_proc.h	5 Apr 2006 22:11:55 -0000	1.405
> --- src/include/catalog/pg_proc.h	23 Apr 2006 02:26:33 -0000
> ***************
> *** 1614,1619 ****
> --- 1614,1625 ----
>   DESCR("convert time with time zone and date to timestamp with time zone");
>   DATA(insert OID = 1299 (  now			   PGNSP PGUID 12 f f t f s 0 1184 "" _null_ _null_ _null_	now - _null_ ));
>   DESCR("current transaction time");
> + DATA(insert OID = 2647 (  transaction_timestamp	PGNSP PGUID 12 f f t f s 0 1184 "" _null_ _null_ _null_	now - _null_ ));
> + DESCR("current transaction time");
> + DATA(insert OID = 2648 (  statement_timestamp	PGNSP PGUID 12 f f t f s 0 1184 "" _null_ _null_ _null_	statement_timestamp - _null_ ));
> + DESCR("current statement time");
> + DATA(insert OID = 2649 (  clock_timestamp	PGNSP PGUID 12 f f t f v 0 1184 "" _null_ _null_ _null_	clock_timestamp - _null_ ));
> + DESCR("current clock time");
>   
>   /* OIDS 1300 - 1399 */
>   
> Index: src/include/utils/timestamp.h
> ===================================================================
> RCS file: /cvsroot/pgsql/src/include/utils/timestamp.h,v
> retrieving revision 1.59
> diff -c -c -r1.59 timestamp.h
> *** src/include/utils/timestamp.h	6 Mar 2006 22:49:17 -0000	1.59
> --- src/include/utils/timestamp.h	23 Apr 2006 02:26:36 -0000
> ***************
> *** 284,289 ****
> --- 284,291 ----
>   extern Datum timestamptz_part(PG_FUNCTION_ARGS);
>   
>   extern Datum now(PG_FUNCTION_ARGS);
> + extern Datum statement_timestamp(PG_FUNCTION_ARGS);
> + extern Datum clock_timestamp(PG_FUNCTION_ARGS);
>   
>   extern Datum pgsql_postmaster_start_time(PG_FUNCTION_ARGS);
>   

> 
> ---------------------------(end of broadcast)---------------------------
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> 
>                http://www.postgresql.org/docs/faq

-- 
  Bruce Momjian   http://candle.pha.pa.us
  EnterpriseDB    http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +

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