Re: [PATCHES] [PATCH] Proposed: Have SPI_connect fail if there is no current snapshot
Martijn van Oosterhout writes: > You can lock a table even while you have no valid snapshot? Certainly. In serializable mode, you really need to be able to take out locks before the snapshot is set --- TFM explains: Note also that if one is relying on explicit locking to prevent concurrent changes, one should use Read Committed mode, or in Serializable mode be careful to obtain the lock(s) before performing queries. A lock obtained by a serializable transaction guarantees that no other transactions modifying the table are still running, but if the snapshot seen by the transaction predates obtaining the lock, it may predate some now-committed changes in the table. A serializable transaction's snapshot is actually frozen at the start of its first query or data-modification command (SELECT, INSERT, UPDATE, or DELETE), so it's possible to obtain locks explicitly before the snapshot is frozen. regards, tom lane ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [PATCHES] [PATCH] Proposed: Have SPI_connect fail if there is no current snapshot
On Sat, Aug 13, 2005 at 06:22:06PM -0400, Tom Lane wrote: > This strikes me as a pretty unreasonable restriction. It would be OK > if there were no valid uses of SPI that didn't require a snapshot, but > that's not so. As an example, consider trying to issue a LOCK TABLE > command via SPI (okay, there are other ways to do that, but it's still > a valid example). You can lock a table even while you have no valid snapshot? I thought that without a valid snapshot you couldn't do anything, except access system tables which are exempt from the usual snapshot rules. In that case it may be better to document the failure case in SPI_exec. -- Martijn van Oosterhout http://svana.org/kleptog/ > Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a > tool for doing 5% of the work and then sitting around waiting for someone > else to do the other 95% so you can sue them. pgpvmc0hHpzS5.pgp Description: PGP signature
Re: [PATCHES] [PATCH] Proposed: Have SPI_connect fail if there is no current snapshot
Martijn van Oosterhout writes: > As per discussion on -hackers, type input functions can be called prior > to there being a current snapshot, causing any queries you execute to > fail with the "no snapshot has been set" error. So I propose to simply > have SPI_connect fail right off the bat and document that to avoid > future surprises. This strikes me as a pretty unreasonable restriction. It would be OK if there were no valid uses of SPI that didn't require a snapshot, but that's not so. As an example, consider trying to issue a LOCK TABLE command via SPI (okay, there are other ways to do that, but it's still a valid example). regards, tom lane ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
[PATCHES] [PATCH] Proposed: Have SPI_connect fail if there is no current snapshot
[Please CC any replies]
Hi,
As per discussion on -hackers, type input functions can be called prior
to there being a current snapshot, causing any queries you execute to
fail with the "no snapshot has been set" error. So I propose to simply
have SPI_connect fail right off the bat and document that to avoid
future surprises. At least until the issue of non-immutable type input
functions has been sorted out.
Have a nice day,
--
Martijn van Oosterhout http://svana.org/kleptog/
> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.
Index: src/backend/executor/spi.c
===
RCS file: /projects/cvsroot/pgsql/src/backend/executor/spi.c,v
retrieving revision 1.131
diff -u -r1.131 spi.c
--- src/backend/executor/spi.c 13 Oct 2004 01:25:10 - 1.131
+++ src/backend/executor/spi.c 13 Aug 2005 19:50:40 -
@@ -70,6 +70,12 @@
*/
if (_SPI_curid != _SPI_connected)
return SPI_ERROR_CONNECT;
+
+ /* You can't actually execute queries if there is no current
+* snapshot, so return an error straightaway. eg User defined type
+* input functions */
+ if (SerializableSnapshot == NULL)
+ return SPI_ERROR_CONNECT;
if (_SPI_stack == NULL)
{
Index: doc/src/sgml/spi.sgml
===
RCS file: /projects/cvsroot/pgsql/doc/src/sgml/spi.sgml,v
retrieving revision 1.42
diff -u -r1.42 spi.sgml
--- doc/src/sgml/spi.sgml 29 Jul 2005 13:00:03 - 1.42
+++ doc/src/sgml/spi.sgml 13 Aug 2005 19:50:40 -
@@ -106,6 +106,13 @@
SPI_finish are forbidden.
(But see SPI_push and SPI_pop.)
+
+
+ SPI_connect will also fail if there is no current
+ snapshot. This generally only happens in implicitly called functions such
+ as type input functions.
+
+
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