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Adjust cost_merge_append() to reflect use of binaryheap_replace_first().

Commit7a2fe9b improved merge append so that replacement of a tupletakes log(N) operations, not twice log(N). Since cost_merge_append knewabout that explicitly, we should adjust it. This probably makes littledifference in practice, but the obsolete comment is confusing.Ideally this would have been put in in 9.3 with the underlying behaviorchange; but I'm not going to back-patch it, since there's some small chanceof changing a plan choice that somebody's optimized for.Thomas MunroDiscussion: <CAEepm=0WQBSvuYcMOUj4Ga4NXpu2J=ejZcE=e=eiTjTX-6_gDw@mail.gmail.com>

1 parent86d19d2 commit34ca090Copy full SHA for 34ca090

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src/backend/optimizer/path
- costsize.c

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`‎src/backend/optimizer/path/costsize.c`

Lines changed: 2 additions & 3 deletions

Original file line number	Diff line number	Diff line change
`@@ -1577,8 +1577,7 @@ cost_sort(Path path, PlannerInfo root,`
`1577`	`1577`	`* at any given instant holds the next tuple from each stream. If there`
`1578`	`1578`	`* are N streams, we need about N*log2(N) tuple comparisons to construct`
`1579`	`1579`	`* the heap at startup, and then for each output tuple, about log2(N)`
`1580`		`- * comparisons to delete the top heap entry and another log2(N) comparisons`
`1581`		`- * to insert its successor from the same stream.`
	`1580`	`+ * comparisons to replace the top entry.`
`1582`	`1581`	`*`
`1583`	`1582`	`* (The effective value of N will drop once some of the input streams are`
`1584`	`1583`	`* exhausted, but it seems unlikely to be worth trying to account for that.)`
`@@ -1619,7 +1618,7 @@ cost_merge_append(Path path, PlannerInfo root,`
`1619`	`1618`	`startup_cost+=comparison_costNlogN;`
`1620`	`1619`
`1621`	`1620`	`/* Per-tuple heap maintenance cost */`
`1622`		`-run_cost+=tuplescomparison_cost2.0*logN;`
	`1621`	`+run_cost+=tuplescomparison_costlogN;`
`1623`	`1622`
`1624`	`1623`	`/*`
`1625`	`1624`	`* Also charge a small amount (arbitrarily set equal to operator cost) per`

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Commit34ca090

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`‎src/backend/optimizer/path/costsize.c`

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