От: Tom Lane
Тема: Re: Weird query speed
Дата: ,
Msg-id: 22803.1074184846@sss.pgh.pa.us
(см: обсуждение, исходный текст)
Ответ на: Weird query speed  ("Rigmor Ukuhe")
Список: pgsql-performance

"Rigmor Ukuhe" <> writes:
> query: select "NP_ID" from a WHERE "NP_ID" > '0'  [is slow]
>
> query: select "NP_ID" from a WHERE "NP_ID" > '1'  [is fast]
>
> There are about 37K rows and only about 100 of then are not "NP_ID" = 0

Yeah, it's scanning over all the zero values when you say "> 0" :-(

This is fixed for 7.5:

2003-12-20 20:23  tgl

    * src/: backend/access/nbtree/nbtinsert.c,
    backend/access/nbtree/nbtpage.c, backend/access/nbtree/nbtsearch.c,
    include/access/nbtree.h: Improve btree's
    initial-positioning-strategy code so that we never need to step
    more than one entry after descending the search tree to arrive at
    the correct place to start the scan.  This can improve the behavior
    substantially when there are many entries equal to the chosen
    boundary value.  Per suggestion from Dmitry Tkach, 14-Jul-03.

            regards, tom lane


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