Re: General performance questions about postgres on Apple

Sean Shanny <shannyconsulting(at)earthlink(dot)net> writes:
> New results with the above changes: (Rather a huge improvement!!!)
> Thanks Scott. I will next attempt to make the cpu_* changes to see if
> it the picks the correct plan.

> explain analyze SELECT t1.id, t2.md5, t2.url from referral_temp t2 LEFT
> OUTER JOIN d_referral t1 ON t2.md5 = t1.referral_md5;
> QUERY PLAN
> ----------------------------------------------------------------------------------------------------------------------------------------------
> Hash Left Join (cost=1669281.60..3204008.48 rows=480082 width=149)
> (actual time=157221.125..-412311.378 rows=502347 loops=1)
> Hash Cond: ("outer".md5 = "inner".referral_md5)
> -> Seq Scan on referral_temp t2 (cost=0.00..16034.81 rows=480081
> width=145) (actual time=11.537..1852.336 rows=502347 loops=1)
> -> Hash (cost=1356358.48..1356358.48 rows=30344048 width=40)
> (actual time=157187.530..157187.530 rows=0 loops=1)
> -> Seq Scan on d_referral t1 (cost=0.00..1356358.48
> rows=30344048 width=40) (actual time=14.134..115048.285 rows=27908024
> loops=1)
> Total runtime: 212595.909 ms
> (6 rows)

It seems like the planner is overestimating the cost of a seqscan
relative to indexed access. Note that the above large seqscan is priced
at 1356358.48 cost units vs 115048.285 actual msec, which says that a
sequential page fetch is taking about 0.1 msec on your hardware.
(You should check the actual size of d_referral to verify this, though.)
The other plan made it look like an indexed fetch was costing several
milliseconds. You may have a situation where you need to raise
random_page_cost, rather than lowering it as people more often do.

What are you using for random_page_cost anyway? It doesn't look like
you are at the default.

This also suggests that the performance issue with your RAID array
has to do with seek time rather than transfer bandwidth...

regards, tom lane