From: Peter Galbavy <peter.galbavy@knowtion.net>
To: pgsql-sql@postgresql.org
Subject: 'fake' join and performance ?
Date: Tue, 22 Oct 2002 17:10:31 +0100
Message-ID: <054501c279e5$8bfcfe00$4528a8c0@cblan.mblox.com> (raw)
OK, I am now confused; postgresql 7.3beta2 on OpenBSD:
photos=# select * from metadata WHERE name = 'Make' and value = 'Canon'
limit 10;
*bang*, 10 values, sub second response.
photos=# select * from metadata m, images i WHERE m.name = 'Make' and
m.value = 'Canon' limit 10;
*yawn* - see you later...
Now, 'images' is a new and currently empty table that I intend to do a join
on later, but I started building a query to test my join'ing skills and
found this;
Explain'ing for both:
photos=# explain select * from metadata WHERE name = 'Make' and value =
'Canon' limit 10;
QUERY PLAN
----------------------------------------------------------------------------
----------------
Limit (cost=0.00..27711.98 rows=6 width=92)
-> Index Scan using metadata_index_2 on metadata (cost=0.00..31072.94
rows=7 width=92)
Index Cond: (name = 'Make'::text)
Filter: (value = 'Canon'::text)
(4 rows)
photos=# explain select * from metadata m, images i WHERE m.name = 'Make'
and m.value = 'Canon' limit 10;
QUERY PLAN
----------------------------------------------------------------------------
------------------------
Limit (cost=0.00..27712.04 rows=6 width=816)
-> Nested Loop (cost=0.00..31073.00 rows=7 width=816)
-> Index Scan using metadata_index_2 on metadata m
(cost=0.00..31072.94 rows=7 width=92)
Index Cond: (name = 'Make'::text)
Filter: (value = 'Canon'::text)
-> Seq Scan on images i (cost=0.00..0.00 rows=1 width=724)
(6 rows)
Er, what's that nested loop. I *know* I have shot myself in the foot
somehow, but my initial reaction was that the optimiser should just make the
'fake' (i.e. unreferenced) reference to another table go away...
peter
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To: pgsql-sql@postgresql.org
Cc: peter.galbavy@knowtion.net
Subject: Re: 'fake' join and performance ?
In-Reply-To: <054501c279e5$8bfcfe00$4528a8c0@cblan.mblox.com>
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