Martha Stewart called it a Good Thing when pkalva@deg.cc (Pallav Kalva) wrote:
>> Then you have to look at individual slow queries to determine why
>> they are slow, fortunately you are running 7.4 so you can set
>> log_min_duration to some number like 1000ms and then
>> try to analyze why those queries are slow.
>
> I had that already set on my database , and when i look at the log
> for all the problem queries, most of the queries are slow from one of
> the table. when i look at the stats on that table they are really
> wrong, not sure how to fix them. i run vacuumdb and analyze daily.
Well, it's at least good news to be able to focus attention on one
table, rather than being unfocused.
If the problem is that stats on one table are bad, then the next
question is "Why is that?"
A sensible answer might be that the table is fairly large, but has
some fields (that are relevant to indexing) that have a small number
of values where some are real common and others aren't.
For instance, you might have a customer/supplier ID where there are
maybe a few hundred unique values, but where the table is dominated by
a handful of them.
The default in PostgreSQL is to collect a histogram of statistics
based on having 10 "bins," filling them using 300 samples. If you
have a pretty skewed distribution on some of the fields, that won't be
good enough.
I would suggest looking for columns where things are likely to be
"skewed" (customer/supplier IDs are really good candidates for this),
and bump them up to collect more stats.
Thus, something like:
alter table my_table alter column something_id set statistics 100;
Then ANALYZE MY_TABLE, which will collect 100 bins worth of stats for
the 'offending' column, based on 3000 sampled records, and see if that
helps.
>> Also hyperthreading may not be helping you..
>
> does it do any harm to the system if it is hyperthreaded ?
Yes. If you have multiple "hyperthreads" running on one CPU, that'll
wind up causing extra memory contention of one sort or another.
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