From: | Philip Warner <pjw(at)rhyme(dot)com(dot)au> |
---|---|
To: | pgsql-bugs(at)postgresql(dot)org |
Subject: | Is this planner choice easily explained? |
Date: | 2002-11-21 11:02:42 |
Message-ID: | 5.1.0.14.0.20021121220031.052d5278@mail.rhyme.com.au |
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Lists: | pgsql-bugs |
This is a summary from a thread on the dbmail mailing list.
I am trying to understand some apparently odd behaviour with 7.2.1 and
7.2.3. Adding a LIMIT statement causes what seems like a very bad choice of
strategy. The first statement is fine:
explain SELECT messageblk FROM messageblks WHERE message_idnr =
100::bigint
ORDER BY messageblk_idnr ;
gives:
Sort (cost=5793.33..5793.33 rows=1453 width=40)
-> Index Scan using messageblks_msg_idx on messageblks
(cost=0.00..5716.99 rows=1453 width=40)
and returns almost instantly, whereas, just adding a limit:
explain SELECT messageblk FROM messageblks WHERE message_idnr =
100::bigint
ORDER BY messageblk_idnr
limit 1;
gives:
Limit (cost=0.00..777.50 rows=1 width=40)
-> Index Scan using messageblks_id_idx on messageblks
(cost=0.00..1129984.15 rows=1453 width=40)
which takes several minutes to run.
The relevant metadata is:
Table "messageblks"
Column | Type | Modifiers
-----------------+--------
+-------------------------------------------------------
messageblk_idnr | bigint | not null default
nextval('messageblk_idnr_seq'::text)
message_idnr | bigint | not null default '0'
messageblk | text | not null
blocksize | bigint | not null default '0'
Indexes: messageblks_msg_idx
Primary key: messageblks_pkey
Unique keys: messageblks_id_idx
Index "messageblks_id_idx"
Column | Type
-----------------+--------
messageblk_idnr | bigint
unique btree
Index "messageblks_msg_idx"
Column | Type
--------------+--------
message_idnr | bigint
btree
If anyone could explain the likely reasons for the choice, I would be very
interested. Even given the planner estimates, they don't look like sensible
choices.
----------------------------------------------------------------
Philip Warner | __---_____
Albatross Consulting Pty. Ltd. |----/ - \
(A.B.N. 75 008 659 498) | /(@) ______---_
Tel: (+61) 0500 83 82 81 | _________ \
Fax: (+61) 03 5330 3172 | ___________ |
Http://www.rhyme.com.au | / \|
| --________--
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and from pgp5.ai.mit.edu:11371 |/
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