About bug #6579
I've looked into this:
http://archives.postgresql.org/pgsql-bugs/2012-04/msg00058.php
and concluded that it's not very practical to fix it properly
right now. A real fix will involve rearranging things so that
construction of the filter-condition list happens at Path creation
time, not createplan time, and that's a rather invasive change.
So I want to put it off until 9.3.
However, I did think of a simple one-line hack we could apply to mask
the worst effects of the bogus estimate, which is just to clamp the
correction factor from the indexquals to be not more than the original
cost estimate for the baserestrict quals, at line 461 in HEAD's
costsize.c:
- cpu_per_tuple -= index_qual_cost.per_tuple;
+ cpu_per_tuple -= Min(index_qual_cost.per_tuple,
+ baserel->baserestrictcost.per_tuple);This seems safe and back-patchable.
regards, tom lane
I wrote:
I've looked into this:
http://archives.postgresql.org/pgsql-bugs/2012-04/msg00058.php
and concluded that it's not very practical to fix it properly
right now. A real fix will involve rearranging things so that
construction of the filter-condition list happens at Path creation
time, not createplan time, and that's a rather invasive change.
So I want to put it off until 9.3.
... and on still further review, I've concluded that this isn't that
expensive to fix locally after all, at least in HEAD; and we get the
further benefit of saner costing of join cases. (As per attached.
Basically it'll cost us one list_difference_ptr operation per IndexPath,
on what will typically be pretty short lists. The cost_qual_eval
operation should be negligible either way, because it will be hitting
RestrictInfo nodes with already-cached costs.)
I'm still inclined to put the quick Min() hack into older branches,
though. While this larger fix could possibly be back-patched, it might
change cost estimates by enough to destabilize plan choices. Given
the small number of complaints about the issue to date, it doesn't seem
worth taking any risk for in released branches.
regards, tom lane