[PATCHES] Buglet in Sort Method explain output in degenerate case
I noticed a small bug in the Sort Method code: postgres=# explain analyze select * from test order by random() limit 1; QUERY PLAN - Limit (cost=21.50..21.50 rows=1 width=4) (actual time=3.649..3.651 rows=1 loops=1) - Sort (cost=21.50..24.00 rows=1000 width=4) (actual time=3.646..3.646 rows=1 loops=1) Sort Key: (random()) Sort Method: quicksort Memory: 17kB - Seq Scan on test (cost=0.00..16.50 rows=1000 width=4) (actual time=0.021..1.707 rows=1000 loops=1) Total runtime: 3.704 ms (6 rows) It's printing quicksort even though it used a heap. This happens because we don't bother deheapifying a singleton heap so the boundUsed flag never gets set. The patch below just moves setting that flag to when the heap is made instead of when it's deheapified. One could make the argument that we should distinguish the noop sort from quicksort or the half-hearted singleton heapsort from the full heapsort but that seems like gilding. But distinguishing between heapsort and quicksort is important since it really could be either depending on how many inputs there were. Index: src/backend/utils/sort/tuplesort.c === RCS file: /home/stark/src/REPOSITORY/pgsql/src/backend/utils/sort/tuplesort.c,v retrieving revision 1.77 diff -u -r1.77 tuplesort.c --- src/backend/utils/sort/tuplesort.c 7 Jun 2007 19:19:57 - 1.77 +++ src/backend/utils/sort/tuplesort.c 1 Sep 2007 17:17:25 - @@ -2247,6 +2247,7 @@ } Assert(state-memtupcount == state-bound); + state-boundUsed = true; state-status = TSS_BOUNDED; } @@ -2284,7 +2285,6 @@ REVERSEDIRECTION(state); state-status = TSS_SORTEDINMEM; - state-boundUsed = true; } /* -- Gregory Stark EnterpriseDB http://www.enterprisedb.com ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [PATCHES] Buglet in Sort Method explain output in degenerate case
Gregory Stark [EMAIL PROTECTED] writes: It's printing quicksort even though it used a heap. This happens because we don't bother deheapifying a singleton heap so the boundUsed flag never gets set. The patch below just moves setting that flag to when the heap is made instead of when it's deheapified. Hmm. Actually, given that sort_bounded_heap() is only conditionally invoked, *both* of the state updates it makes are bogus. But I think they should both be done at the call site in tuplesort_performsort. (The state-status update already is, which is why it works at all.) Setting it at conclusion is correct, I think, since if we ever changed the code to abandon TSS_BOUNDED state in the face of unexpected memory growth, it would be wrong to have set it in make_bounded_sort. regards, tom lane ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PATCHES] Buglet in Sort Method explain output in degenerate case
I wrote: Hmm. Actually, given that sort_bounded_heap() is only conditionally invoked, *both* of the state updates it makes are bogus. Er, make that three state updates: its REVERSEDIRECTION() operation is being skipped as well. That's not critical now, but might be someday. Rather than moving all that up to tuplesort_performsort, it seems better to leave it where it is, and instead remove the premature optimization of trying to skip sort_bounded_heap. The number of cycles saved that way is tiny anyway... regards, tom lane ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
