JB <jimbag@kw.igs.net> writes:
> My apologies for chewing up bandwidth.
Not at all! Just because I don't understand it does not mean
you haven't found an effect worth looking into ;-)
> I'm running 6.5.2 on RH6.1, 128mb ram, 27gb, P350.
OK, cool. We've had a couple of weird-looking questions that turned
out to be from people running ancient releases, so "what version" is
something we all routinely ask now.
> ---[snip]---
> #!/bin/sh
> psql -c "EXPLAIN SELECT * FROM info WHERE substring(stname from 1 for 4)
> = 'MAIN';"
> time psql -c "SELECT * FROM info WHERE substring(stname from 1 for 4) =
> 'MAIN';"
> psql -c "EXPLAIN SELECT * FROM info WHERE stname LIKE 'MAIN%';"
> time psql -c "SELECT * FROM info WHERE stname LIKE 'MAIN%';"
> ---[snip]---
> outputs...
> Seq Scan on info (cost=3829.93 rows=15454 width=420)
> 0.01user 0.01system 0:00.72elapsed 2%CPU (0avgtext+0avgdata
> 0maxresident)k
> 0inputs+0outputs (198major+25minor)pagefaults 0swaps
> Index Scan using nx_info1 on info (cost=1531.12 rows=30 width=420)
> 0.01user 0.01system 0:00.64elapsed 3%CPU (0avgtext+0avgdata
> 0maxresident)k
> 0inputs+0outputs (198major+25minor)pagefaults 0swaps
> Obviously the numbers don't support me. I'm quite confused.
"time psql" doesn't really tell you anything much, since the CPU
numbers it cites only cover the psql front end, not the backend
database server. You can put some faith in the "elapsed time"
values, but only if your machine is otherwise idle. In this case
you have readings 0.72 and 0.64, which are IMHO too close to call;
you'd need to make a longer-running test case to have much confidence
in the results.
But you said before that you saw 20 sec vs. 2 sec, which is surely
a significant difference (barring major load variations from other
programs on your machine); can you duplicate that?
> I was told that the engine didn't use indexes with 'LIKE' by someone
> equally informed as I, and thus the 'substring' change.
Postgres does use an index for "foo LIKE 'bar%'" if it can. 6.5
is not very bright about this when you have USE_LOCALE enabled,
but 7.0 is smarter.
> There must be something with the bigger system that I need to
> look into (mem usage, etc).
It's worth looking into. Feel free to contact me off-list if you
want to probe further.
regards, tom lane
