Re: COUNT and Performance ...
От | Neil Conway |
---|---|
Тема | Re: COUNT and Performance ... |
Дата | |
Msg-id | 1044217919.25210.13.camel@tokyo обсуждение исходный текст |
Ответ на | Re: COUNT and Performance ... (Tom Lane <tgl@sss.pgh.pa.us>) |
Список | pgsql-hackers |
On Sun, 2003-02-02 at 13:04, Tom Lane wrote: > I think your test case is small enough that the whole table is resident > in memory, so this measurement only accounts for CPU time per tuple and > not any I/O. Given the small size of pgstattuple's per-tuple loop, the > speed differential is not too surprising --- but it won't scale up to > larger tables. Good observation. When the entire table is in cache, pgstattuple about 4 times faster than count(*) on my machine. When the table is too large to fit into cache, the performance difference drops to 8% in favour of pgstattuple: nconway=# select count(*) from big_table; count ---------8388612 (1 row) Time: 26769.99 ms nconway=# SELECT tuple_count FROM pgstattuple('big_table');tuple_count ------------- 8388612 (1 row) Time: 24658.87 ms Cheers, Neil -- Neil Conway <neilc@samurai.com> || PGP Key ID: DB3C29FC
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