* Tom Lane (t...@sss.pgh.pa.us) wrote: > > What I'm trying to get at in this overall email is: why in the world is > > it so expensive to do hash lookups? > > perf or oprofile reveal anything?
Working on a test case actually- I've got one now: http://snowman.net/~sfrost/test_case2.sql In this example, hashing the large table is actually 2 seconds *faster* than hashing the small table (again, all on my laptop). > Also, I assume that the cases you are looking at are large enough that > even the "small" table doesn't fit in a single hash batch? No, quite the opposite, sorry for not mentioning that before. Either side fits completely into memory w/ a single batch. The explain analyze's that I posted before show that, either way, there's only one batch involved. > (You never did mention what work_mem setting you're testing, anyway.) With the test case above (where I got a 2s faster run time by hashing the big table) used a work_mem of 1GB. Thanks! Stephen
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