Hello. At Wed, 10 Jul 2019 15:02:57 +0900 (Tokyo Standard Time), Kyotaro Horiguchi <horikyota....@gmail.com> wrote in <20190710.150257.260806103.horikyota....@gmail.com> > It is seen by a simpler test. > > create table t as select a from generate_series(0, 99999) a; > analyze t; > explain analyze select * from t order by a desc; > Execution Time: 116.613 ms > explain analyze select * from t order by a desc fetch first 1 percent rows > only; > Execution Time: 158.458 ms > explain analyze select * from t order by a desc fetch first 100 percent rows > only; > Execution Time: 364.442 ms > > I didn't looked closer to the version. Fetching from tuplestore > and returning all tuples costs 206ms and it is exceeding the cost > of fething of the whole table and returning all tuples. I don't > believe tuplestore that isn't splling out to disk is so slower > than (cached) table access. > > Other than that, we can rip the clause if it is 100%
As a more significant point, I found that the first query in the aboves runs faster by about 10-18% on master(unpatched). explain analyze select * from t order by a desc; Execution Time: 96.690 ms But perf didn't give me useful information. patched: 11857 11.7065 postgres qsort_ssup 9026 8.9114 postgres ApplySortComparator 6443 6.3612 [vdso] (tgid:8388 range:0x7ffed49ed000-0x7ffed49eefff) [vdso] (tgid:8388 range:0x7ffed49ed000-0x7ffed49eefff) 5826 5.7520 postgres btint4fastcmp 4699 4.6393 no-vmlinux /no-vmlinux 3451 3.4072 libc-2.17.so __memcpy_ssse3_back 3270 3.2285 postgres LogicalTapeWrite 2972 2.9343 postgres copytup_heap 2961 2.9234 postgres readtup_heap 2769 2.7338 postgres LogicalTapeRead 2457 2.4258 postgres GetMemoryChunkContext 2147 2.1197 postgres InstrStopNode 2021 1.9953 postgres heapgettup_pagemode 1583 1.5629 postgres writetup_heap 1555 1.5353 postgres tuplesort_gettuple_common 1508 1.4889 postgres AllocSetAlloc ... master: 12932 12.0168 postgres qsort_ssup 9491 8.8193 postgres ApplySortComparator 6705 6.2305 postgres btint4fastcmp 6557 6.0930 [vdso] (tgid:6341 range:0x7ffdd0315000-0x7ffdd0316fff) [vdso] (tgid:6341 range:0x7ffdd0315000-0x7ffdd0316fff) 4874 4.5291 no-vmlinux /no-vmlinux 4059 3.7717 postgres readtup_heap 3707 3.4447 libc-2.17.so __memcpy_ssse3_back 3583 3.3294 postgres LogicalTapeWrite 3382 3.1427 postgres LogicalTapeRead 3001 2.7886 postgres copytup_heap 2522 2.3435 postgres GetMemoryChunkContext 2464 2.2896 postgres heapgettup_pagemode 2115 1.9653 postgres InstrStopNode 1847 1.7163 postgres tuplesort_gettuple_common 1652 1.5351 postgres writetup_heap 1565 1.4542 postgres AllocSetAlloc regards. -- Kyotaro Horiguchi NTT Open Source Software Center
