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

2015年8月22日 00:32:11 +0000 (20:32 -0400)

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

src/backend/executor/spi.c patch | blob | blame | history

diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c

index d544ad9c1062cf0ba486eabe6ce7b7ebc5b66a50..300401e01f98424fadf8aaf6608791f24a600c1b 100644 (file)

--- a/src/backend/executor/spi.c

+++ b/src/backend/executor/spi.c

@@ -1797,7 +1797,8 @@ spi_printtup(TupleTableSlot *slot, DestReceiver *self)

if (tuptable->free == 0)

{

- tuptable->free = 256;

+ /* Double the size of the pointer array */

+ tuptable->free = tuptable->alloced;

tuptable->alloced += tuptable->free;

tuptable->vals = (HeapTuple *) repalloc(tuptable->vals,

tuptable->alloced * sizeof(HeapTuple));

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