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Fix excessive memory consumption in the new sort pre-reading code.

LogicalTapeRewind() should not allocate large read buffer, if the tapeis completely empty. The calling code relies on that, for itscalculation of how much memory to allocate for the read buffers. Thatlead to massive overallocation of memory, if maxTapes was high, butonly a few tapes were actually used.Reported by Tomas VondraDiscussion: <7303da46-daf7-9c68-3cc1-9f83235cf37e@2ndquadrant.com>

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src/backend/utils/sort
- logtape.c

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`‎src/backend/utils/sort/logtape.c`

Lines changed: 8 additions & 3 deletions

Original file line number	Diff line number	Diff line change
`@@ -778,11 +778,16 @@ LogicalTapeRewind(LogicalTapeSet *lts, int tapenum, bool forWrite)`
`778`	`778`	`datablocknum=ltsRewindFrozenIndirectBlock(lts,lt->indirect);`
`779`	`779`	`}`
`780`	`780`
`781`		`-/* Allocate a read buffer */`
	`781`	`+/* Allocate a read buffer(unless the tape is empty)*/`
`782`	`782`	`if (lt->buffer)`
`783`	`783`	`pfree(lt->buffer);`
`784`		`-lt->buffer=palloc(lt->read_buffer_size);`
`785`		`-lt->buffer_size=lt->read_buffer_size;`
	`784`	`+lt->buffer=NULL;`
	`785`	`+lt->buffer_size=0;`
	`786`	`+if (datablocknum!=-1L)`
	`787`	`+{`
	`788`	`+lt->buffer=palloc(lt->read_buffer_size);`
	`789`	`+lt->buffer_size=lt->read_buffer_size;`
	`790`	`+}`
`786`	`791`
`787`	`792`	`/* Read the first block, or reset if tape is empty */`
`788`	`793`	`lt->curBlockNumber=0L;`

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Commitb56fb69

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`‎src/backend/utils/sort/logtape.c`

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