You might be able to improve matters by increasing the statistics target
for this table.
I have tried to increase the statistics for the status column to the maximum
of 1000. After that I performed an analyze, vacuum analyze and vacuum full
analyze on the table. Unfortunately this didn't seem to make any difference.
I have a bad feeling though that the problem may be
lack of cross-column statistics
I assume this isn't a thing that can be tweaked/increased in PG 8.2?
--- the thing is evidently assuming
that only about 1 in 200 rows have status = '0', which might be accurate
as a global average but not for this particular merchant. What exactly
is the relationship between status and merchant_id, anyway?
The meaning is that a "banners_link" belongs to a merchant with the id
merchant_id. A "banners_link" can be disabled (status 1) or enabled (status
0). Globally about 1/3 of the banners_links have status 0 and 2/3 have
status 1. The 1 in 200 estimate is indeed way off.
For the merchant in question the numbers are a bit different though, but not
that much. Out of 424 rows total, 359 have status 0 and 65 have status 1.
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TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
