From: | Pailloncy Jean-Gérard <pailloncy(at)ifrance(dot)com> |
---|---|
To: | pgsql-performance(at)postgresql(dot)org |
Subject: | Index Backward Scan fast / Index Scan slow ! |
Date: | 2004-04-12 09:26:55 |
Message-ID: | 8A2105D4-8C63-11D8-80FD-000A95DE2550@ifrance.com |
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Thread: | |
Lists: | pgsql-performance |
Hi,
I test many times the foolowing query.
dps=# explain analyze select next_index_time from url order by
next_index_time desc limit 1;
QUERY PLAN
------------------------------------------------------------------------
------------------------------------------------------------------------
----
Limit (cost=0.00..2.62 rows=1 width=4) (actual time=56.615..56.616
rows=1 loops=1)
-> Index Scan Backward using url_next_index_time on url
(cost=0.00..768529.55 rows=293588 width=4) (actual time=56.610..56.610
rows=1 loops=1)
Total runtime: 56.669 ms
(3 rows)
dps=# explain analyze select next_index_time from url order by
next_index_time asc limit 1;
QUERY PLAN
------------------------------------------------------------------------
------------------------------------------------------------------------
-
Limit (cost=0.00..2.62 rows=1 width=4) (actual
time=94879.636..94879.637 rows=1 loops=1)
-> Index Scan using url_next_index_time on url
(cost=0.00..768529.55 rows=293588 width=4) (actual
time=94879.631..94879.631 rows=1 loops=1)
Total runtime: 94879.688 ms
(3 rows)
How to optimize the last query ? (~ 2000 times slower than the first
one)
I suppose there is some odd distribution of data in the index ?
Is the solution to reindex data ?
Cordialement,
Jean-Gérard Pailloncy
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