Hello, hackers.
Yesterday I have noticed that in simple protocol mode snapshot is
taken twice - first time for parsing/analyze and later for execution.
I was thinking it is a great idea to reuse the same snapshot. After
some time (not short) I was able to find this thread from 2011 with
exactly same idea (of course after I already got few % of performance
in POC):
https://www.postgresql.org/message-id/flat/CA%2BTgmoYqKRj9BozjB-%2BtLQgVkSvzPFWBEzRF4PM2xjPOsmFRdw%40mail.gmail.com
And it was even merged: "Take fewer snapshots" (
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=d573e239f03506920938bf0be56c868d9c3416da
)
But where is optimisation in the HEAD?
It absent because was reverted later in 2012 because of tricky reasons
( https://www.postgresql.org/message-id/flat/5075D8DF.6050500%40fuzzy.cz
) in commit "Revert patch for taking fewer snapshots." (
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=532994299e2ff208a58376134fab75f5ae471e41
)
I think it is good idea to add few comments to code related to the
topic in order to safe time for a next guy.
Comments-only patch attached.
Thanks,
Michail.
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 0a6f80963b..81880ddbba 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -1155,7 +1155,13 @@ exec_simple_query(const char *query_string)
plantree_list = pg_plan_queries(querytree_list,
CURSOR_OPT_PARALLEL_OK, NULL);
- /* Done with the snapshot used for parsing/planning */
+ /* Done with the snapshot used for parsing/planning.
+ *
+ * While it is looks promising to reuse snapshot for query
execution (at least for simple protocol)
+ * but unfortunately it caused execution to use a snapshot that
had been acquired before
+ * locking any of the tables mentioned in the query.
+ * This created user-visible anomalies that were not present in
any prior release.
+ */
if (snapshot_set)
PopActiveSnapshot();
