Le 16/03/2010 15:25, Richard Huxton a écrit :
OK - we have a merge join in the first case where it joins the
pre-sorted output of both tables.
In the second case it queries the index once for each row in "cellules".
Now look at the costs. The first one is around 704,000 and the second
one is 5,000,000 - about 6 times as much. That's why it's not using the
index, because it thinks it will be more expensive.
If it's not really more expensive that suggests your configuration
values aren't very close to reality.
The first query should run faster if it has more work_mem available too.
At the moment, it's probably going back and fore doing an on-disk sort.
Indeed !
I admit that I had not tested the second query, I just thought that the
first one took way too long to execute.
I will try increasing work_mem, but it is already set at 16MB which I
found is quite high.
Thanks a lot for clarifying that !
Regards
Arnaud
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