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Avoid O(N^2) behavior when enlarging SPI tuple table in spi_printtup().

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For no obvious reason, spi_printtup() was coded to enlarge the tuple
pointer table by just 256 slots at a time, rather than doubling the size at
each reallocation, as is our usual habit.  For very large SPI results, this
makes for O(N^2) time spent in repalloc(), which of course soon comes to
dominate the runtime.  Use the standard doubling approach instead.

This is a longstanding performance bug, so back-patch to all active
branches.

Neil Conway
This commit is contained in:
Tom Lane 2015-08-21 20:32:11 -04:00
parent fcdfce6820
commit 6e5d9f278c
1 changed files with 2 additions and 1 deletions
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3
src/backend/executor/spi.c
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@ -1797,7 +1797,8 @@ spi_printtup(TupleTableSlot *slot, DestReceiver *self)
if (tuptable->free == 0) if (tuptable->free == 0)
{ {
tuptable->free = 256; /* Double the size of the pointer array */
tuptable->free = tuptable->alloced;
tuptable->alloced += tuptable->free; tuptable->alloced += tuptable->free;
tuptable->vals = (HeapTuple *) repalloc(tuptable->vals, tuptable->vals = (HeapTuple *) repalloc(tuptable->vals,
tuptable->alloced * sizeof(HeapTuple)); tuptable->alloced * sizeof(HeapTuple));