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Take fewer snapshots.

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When a PORTAL_ONE_SELECT query is executed, we can opportunistically
reuse the parse/plan shot for the execution phase.  This cuts down the
number of snapshots per simple query from 2 to 1 for the simple
protocol, and 3 to 2 for the extended protocol.  Since we are only
reusing a snapshot taken early in the processing of the same protocol
message, the change shouldn't be user-visible, except that the remote
possibility of the planning and execution snapshots being different is
eliminated.

Note that this change does not make it safe to assume that the parse/plan
snapshot will certainly be reused; that will currently only happen if
PortalStart() decides to use the PORTAL_ONE_SELECT strategy.  It might
be worth trying to provide some stronger guarantees here in the future,
but for now we don't.

Patch by me; review by Dimitri Fontaine.
This commit is contained in:
Robert Haas
2011-12-21 09:16:55 -05:00
parent e1042a3484
commit d573e239f0
6 changed files with 39 additions and 32 deletions
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18
src/backend/executor/spi.c
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@ -1126,7 +1126,6 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
CachedPlan *cplan;
List *stmt_list;
char *query_string;
Snapshot snapshot;
MemoryContext oldcontext;
Portal portal;
@ -1269,15 +1268,6 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
}
}
/* Set up the snapshot to use. */
if (read_only)
snapshot = GetActiveSnapshot();
else
{
CommandCounterIncrement();
snapshot = GetTransactionSnapshot();
}
/*
* If the plan has parameters, copy them into the portal. Note that this
* must be done after revalidating the plan, because in dynamic parameter
@ -1293,7 +1283,13 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
/*
* Start portal execution.
*/
PortalStart(portal, paramLI, snapshot);
if (read_only)
PortalStart(portal, paramLI, true);
else
{
CommandCounterIncrement();
PortalStart(portal, paramLI, false);
}
Assert(portal->strategy != PORTAL_MULTI_QUERY);