Re: Mostly read performance (2 replies) - Mailing list pgsql-performance

(Pardon my replying two two replies at once, I only get the digest and
this was easier).

Michael Stone wrote:
[...]
>> Well, that's what you'd expect.  But a first time 70MB fetch on a freshly rebooted system took just as long as all
secondarytimes.  (Took over a minute to fetch, which is too long for my needs, at least on secondary attempts). 
>
>
> If the query involves a table scan and the data set is larger than your
> available memory, you'll need a full scan every time. If you do a table
> scan and the table fits in RAM, subsequent runs should be faster. If you
> have an index and only need to look at a subset of the table, subsequent
> runs should be faster. Without knowing more about your queries it's not
> clear what your situation is.

I must amend my original statement.  I'm not using a parameterized
statement.  The system is effectively fetching file content stored in
the database for portions of one or more files.  It attempts to batch
the records being fetched into as few non-parameterized queries as
possible, while balancing the rowset retrieval memory impact.

Currently that means it will request up to 16K records in a query that
is assembled using a combination of IN (recids...) , BETWEEN ranges, and
UNION ALL for multiple file IDs.  I do this to minimize the latency of
dbclient/dbserver requests, while at the same time capping the maximum
data returned by a DBIO to about 1.2MB per maximum retrieved record set.
(I'm trying not to pound the java app server via jdbc memory usage).
There's an ORDER BY on the file id column too.

It sounds like a simple enough thing to do, but this "pieces of many
files in a database" problem is actually pretty hard to optimize.
Fetching all records for all files, even though I don't need all
records, is both inefficient and likely to use too much memory.
Fetching 1 file at a time is likely to result in too many queries
(latency overhead).  So right now I err on the side of large but record
limited queries.  That let's me process many files in one query, unless
the pieces of the files I need are substantial.
(I've been burned by trying to use setFetchSize so many times it isn't
funny, I never count on that any more).

An index is in place to assist with record selection, I'll double check
that it's being used.  It's a joint index on file-id and
record-id-within-the-file.  I'll check to be sure it's being used.

------------------------


Greg Stark wrote:
[...]

> What is your shared_buffers setting? Perhaps you have it set way too high or
> way too low?

I generally run with the conservative installation default.  I did some
experimenting with larger values but didn't see any improvement (and
yes, I restarted postmaster).  This testing was done a while ago, I
don't have the numbers in memory any more so I can't tell you what they
were.

>
> Also, you probably should post the "explain analyze" output of the actual
> query you're trying to optimize. Even if you're not looking for a better plan
> having hard numbers is better than guessing.

A good suggestion.  I'll look into it.


> And the best way to tell if the data is cached is having a "vmstat 1" running
> in another window. Start the query and look at the bi/bo columns. If you see
> bi spike upwards then it's reading from disk.

Another good suggestion.

I'll look into getting further data from the above suggestions.

I'm also looking into getting a gig or two of ram to make sure that
isn't an issue.

The basis of my post originally was to make sure that, all things being
equal, there's no reason those disk I/Os on behalf of the database
shouldn't be cached by the operating/file system so that repeated reads
might benefit from in-memory data.