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Re: query planner and scanning methods

From: "Richard Broersma" <richard(dot)broersma(at)gmail(dot)com>
To: "Colin Copeland" <copelco(at)caktusgroup(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org, "team(at)caktusgroup(dot)com s" <team(at)caktusgroup(dot)com>
Subject: Re: query planner and scanning methods
Date: 2008-09-23 22:07:50
Message-ID: 396486430809231507t46edf20fxab469bedbd78e8cc@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Tue, Sep 23, 2008 at 2:22 PM, Colin Copeland <copelco(at)caktusgroup(dot)com> wrote:
> dimension=# EXPLAIN ANALYZE
> SELECT   DISTINCT ON ("dimension_book"."call")
>         "dimension_book"."title"
> FROM     "dimension_book"
>         INNER JOIN "dimension_library_books"
>           ON ("dimension_book"."id" = "dimension_library_books"."book_id")
> WHERE    ("dimension_book"."call" >= 'PA0000'
>          AND "dimension_library_books"."library_id" IN (12,15,20))
> ORDER BY "dimension_book"."call" ASC
> LIMIT 10 OFFSET 100;

Ya offset works by scanning over the first 100 rows.  When the offsets
get big, it become a performance looser.

You can guarantee a faster index scan if you recall the last 10th
value from the previous query.  Then remove the offset predicate and
replace it with the following WHERE clause:

WHERE ...
AND dimension_book.call > _last_queried_10th_row-dimension_book_call,
...
LIMIT 10;


-- 
Regards,
Richard Broersma Jr.

Visit the Los Angeles PostgreSQL Users Group (LAPUG)
http://pugs.postgresql.org/lapug

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