Re: So, is COUNT(*) fast now? - Mailing list pgsql-hackers

On Fri, Oct 21, 2011 at 2:33 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I think HeapTupleSatisfiesMVCC is probably being skipped anyway in
>> this case, since all the heap pages should be PD_ALL_VISIBLE.
>
> Proves my point ;-) ... you're comparing a code path that's been beat on
> for *years* with one that just got written.

I know.  I wrote a chunk of it.  :-)  My point is just that it'd be
nice to make it better.

Anyhow, here's the scoop.  On my desktop machine running F14, running
SELECT sum(1) FROM pgbench_accounts in a tight loop, 60 s worth of
oprofile data:

176830   13.0801  postgres                 postgres                 ExecProject
170028   12.5770  postgres                 postgres
IndexOnlyNext
96631     7.1478  postgres                 postgres
visibilitymap_test
86019     6.3628  postgres                 postgres
advance_aggregates
74366     5.5009  postgres                 postgres                 ExecScan
72428     5.3575  postgres                 postgres
ExecClearTuple
68483     5.0657  postgres                 postgres                 btgettuple
60614     4.4836  postgres                 postgres
advance_transition_function
59680     4.4145  postgres                 postgres                 ExecProcNode
52295     3.8683  postgres                 postgres
_bt_checkkeys
52078     3.8522  libc-2.12.90.so          libc-2.12.90.so
__memcpy_sse2
49548     3.6651  postgres                 postgres
index_getnext_tid
48265     3.5702  postgres                 postgres
ExecEvalConst
42989     3.1799  postgres                 postgres                 _bt_next
40544     2.9990  postgres                 postgres                 _bt_readpage
35162     2.6009  no-vmlinux               no-vmlinux               /no-vmlinux
33639     2.4883  postgres                 postgres
MemoryContextReset

And without index-only scans. but everything in shared_buffers:

169515   18.4261  postgres                 postgres                 ExecProject
94827    10.3076  postgres                 postgres
heapgettup_pagemode
84850     9.2231  postgres                 postgres
advance_aggregates
57998     6.3043  postgres                 postgres
advance_transition_function
55638     6.0478  postgres                 postgres
ExecEvalConst
53684     5.8354  postgres                 postgres                 heapgetpage
51411     5.5883  postgres                 postgres                 ExecScan
48387     5.2596  postgres                 postgres                 ExecProcNode
44129     4.7968  postgres                 postgres
ExecStoreTuple
30759     3.3435  postgres                 postgres                 heap_getnext
25923     2.8178  postgres                 postgres                 SeqNext
24145     2.6245  postgres                 postgres
CheckForSerializableConflictOut
23155     2.5169  postgres                 postgres                 ExecAgg
18864     2.0505  postgres                 postgres
heap_page_prune_opt
18784     2.0418  no-vmlinux               no-vmlinux               /no-vmlinux

The index-only scan takes about 385 ms, while the non-index-only
version takes about 284 ms.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company