Tom Lane wrote:
Dave Tenny <tenny@attbi.com> writes:
My application relies heavily on IN lists. The lists are primarily
constant integers, so queries look like:
SELECT val FROM table WHERE id IN (43, 49, 1001, 100002, ...)
1) PostgreSQL exhibits worse-than-linear performance behavior with
respect to IN list size.
Yeah. There are a couple of places in the planner that have O(N^2)
behavior on sufficiently large WHERE clauses, due to building lists
in a naive way (repeated lappend() operations). The inner loop is
very tight, but nonetheless when you do it tens of millions of times
it adds up :-(
It was also showing up very clearly in the timings I attached for just a couple hundred IN list entries too.
Does that mean that the time for the O(1) portions of the logic are perhaps also in need of a tuneup?
(I would think O(N^2) for trivial operations on a fast machine wouldn't manifest quite so much
for smallish N).
I have just committed some fixes into CVS tip for this --- I see about
a 10x speedup in planning time on test cases involving 10000-OR-item
WHERE clauses. We looked at this once before; the test cases I'm using
actually date back to Jan 2000. But it seems some slowness has crept
in due to subsequent planning improvements.
So what version might that equate to down the road, so I can be sure to check it out?
I'm afraid I'm not up to testing CVS tips.
4) God help you if you haven't vacuum/analyzed that the newly loaded
table.
Without knowledge that the id field is unique, the planner is likely to
tilt away from an indexscan with not too many IN items. I don't
consider this a bug.
There is one very interesting thing in my test case though. It certainly
seemed as if the
parameterized statements were successfully using the index of the freshly-created-but-unanalyzed table,
or else the times on those queries would have been terrible too. It was only the IN list form
of query that wasn't making correct use of the index. How can the planner recognize uniqueness for
one case but not the other? (Since I ran both forms of query on the same table with and without vacuuming).
PostgreSQL craps out trying to process 8000 elements with the error: out of free buffers: time to abort!
This is a known bug in 7.3.2; it's fixed in 7.3.3.
Thanks, that's good to know.
Dave
