From: Stergios Zissakis <szis@intranet.gr>
To: Interfaces <pgsql-interfaces@postgresql.org>
Cc: Kostas Lykiardopoulos <klyk@intranet.gr>
Cc: Dimitris Pantermalis <dpant@intranet.gr>
Subject: [ECPG] Multiple cursors in the same transaction
Date: Mon, 7 Jun 2004 16:36:29 +0300
Message-ID: <NGBBJHBHDKPNOODGDMMAOECMCIAA.szis@intranet.gr> (raw)
Dear ECPG support,
During a port from Oracle I came accross a problem where a cursor is lost
when a commit is performed. Consider the following piece of code:
EXEC SQL WHENEVER SQLERROR DO call_sql_error_function(...);
EXEC SQL CONNECT TO :connect_str USER :username IDENTIFIED BY :password;
sprintf((char *) del_stmt.arr, "select columns from delete_table;\0");
EXEC SQL PREPARE Delete FROM del_stmt;
EXEC SQL DECLARE del1 CURSOR FOR Delete;
EXEC SQL OPEN del1;
EXEC SQL WHENEVER NOT FOUND DO break;
while( true )
{
EXEC SQL FETCH FROM del1 INTO :dlt;
...
sprintf((char *) stmt.arr, "select to_char((date_trunc('month',
localtimestamp) - interval '%d month'), 'MM/DD/YYYY HH24:MI:SS');\0",
months);
EXEC SQL PREPARE S1 FROM :stmt;
EXEC SQL DECLARE mytime CURSOR FOR S1;
EXEC SQL OPEN mytime;
EXEC SQL FETCH mytime INTO :mydate;
EXEC SQL CLOSE mytime;
...
sprintf((char *) select_stmt, "select ... from ... where ...;\0", ...);
EXEC SQL PREPARE ARCH FROM :select_stmt;
EXEC SQL DECLARE SEL_CURS CURSOR FROM ARCH;
EXEC SQL OPEN SEL_CURS;
EXEC SQL WHENEVER NOT FOUND DO break;
while( true )
{
EXEC SQL FETCH :rows_to_fetch FROM SEL_CURS INTO :alarm_records;
for( int i = 0; i < sqlca.sqlerrd[2]; i++ )
{
...
}
}
EXEC SQL CLOSE SEL_CURS;
...
EXEC SQL WHENVER NOT FOUND goto somewhere;
sprintf((char *) del2_stmt, "delete from %s where %s;\0", from, where);
EXEC SQL PREPARE DEL2 FROM :del2_stmt;
EXEC SQL EXECUTE DEL2;
EXEC SQL COMMIT; // I think this commit causes the problem... see the
comments further down.
somewhere:
...
}
EXEC SQL CLOSE del1;
EXEC SQL COMMIT WORK RELEASE;
The problem is that fetching the second time round from the del1 cursor
cause the app to bomb out. The postmaster log displays the following
message: "... ERROR: cursor del1 does not exist". Any ideas why this is
happening? Who come del1 dissapears whithout closing it? Notice that my
delete_table (del_stmt statement at the beginning) contains a single row; so
the second time a fetch is performed it should simply not found anything and
do a break to terminate the loop.
The work-around I found was to split the work in two different connections
(contexts); one for selecting from the delete_table and the other for the
rest of the statements. This way everything works fine but I cannot really
afford an extra connection for this (my app has already 20 connections by
design). Here is the code that solved the problem:
EXEC SQL WHENEVER SQLERROR DO call_sql_error_function(...);
EXEC SQL CONNECT TO :connect_str AS :ctx1 USER :username IDENTIFIED BY
:password;
EXEC SQL CONNECT TO :connect_str AS :ctx2USER :username IDENTIFIED BY
:password;
sprintf((char *) del_stmt.arr, "select columns from delete_table;\0");
EXEC SQL PREPARE Delete FROM del_stmt;
EXEC SQL AT :ctx1 DECLARE del1 CURSOR FOR Delete;
EXEC SQL AT :ctx1 OPEN del1;
EXEC SQL WHENEVER NOT FOUND DO break;
while( true )
{
EXEC SQL AT :ctx1 FETCH FROM del1 INTO :dlt;
...
sprintf((char *) stmt.arr, "select to_char((date_trunc('month',
localtimestamp) - interval '%d month'), 'MM/DD/YYYY HH24:MI:SS');\0",
months);
EXEC SQL PREPARE S1 FROM :stmt;
EXEC SQL AT :ctx2 DECLARE mytime CURSOR FOR S1;
EXEC SQL AT :ctx2 OPEN mytime;
EXEC SQL AT :ctx2 FETCH mytime INTO :mydate;
EXEC SQL AT :ctx2 CLOSE mytime;
...
sprintf((char *) select_stmt, "select ... from ... where ...;\0", ...);
EXEC SQL PREPARE ARCH FROM :select_stmt;
EXEC SQL AT :ctx2 DECLARE SEL_CURS CURSOR FROM ARCH;
EXEC SQL AT :ctx2 OPEN SEL_CURS;
EXEC SQL WHENEVER NOT FOUND DO break;
while( true )
{
EXEC SQL AT :ctx2 FETCH :rows_to_fetch FROM SEL_CURS INTO
:alarm_records;
for( int i = 0; i < sqlca.sqlerrd[2]; i++ )
{
...
}
}
EXEC SQL AT :ctx2 CLOSE SEL_CURS;
...
EXEC SQL WHENVER NOT FOUND goto somewhere;
sprintf((char *) del2_stmt, "delete from %s where %s;\0", from, where);
EXEC SQL PREPARE DEL2 FROM :del2_stmt;
EXEC SQL AT :ctx2 EXECUTE DEL2;
EXEC SQL AT :ctx2 COMMIT;
somewhere:
...
}
EXEC SQL AT :ctx1 CLOSE del1;
EXEC SQL COMMIT WORK RELEASE;
Is there any other solution to this problem?
Thanks in advance.
Stergios Zissakis
Intracom S.A.
Network Management Systems Dept.
Paiania
Athens
Greece
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To: pgsql-interfaces@postgresql.org
Cc: szis@intranet.gr, klyk@intranet.gr, dpant@intranet.gr
Subject: Re: [ECPG] Multiple cursors in the same transaction
In-Reply-To: <NGBBJHBHDKPNOODGDMMAOECMCIAA.szis@intranet.gr>
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