Re: Slow count(*) again...

On Oct 10, 2010, at 10:14 AM, Mladen Gogala wrote:

>
> SQL> show parameter db_file_multi
>
> NAME TYPE VALUE
> ------------------------------------ -----------
> ------------------------------
> db_file_multiblock_read_count integer 16
> SQL> alter session set db_file_multiblock_read_count=1;
>
> Session altered.
> SQL> select count(*) from ni_occurrence;
>
> COUNT(*)
> ----------
> 402062638
>
> Elapsed: 00:08:20.88
> SQL> alter session set db_file_multiblock_read_count=128;
>
> Session altered.
>
> Elapsed: 00:00:00.50
> SQL> select count(*) from ni_occurrence;
>
> COUNT(*)
> ----------
> 402062638
>
> Elapsed: 00:02:17.58
>
>
> In other words, when I batched the sequential scan to do 128 blocks I/O,
> it was 4 times faster than when I did the single block I/O.
> Does that provide enough of an evidence and, if not, why not?
>

Did you tune the linux FS read-ahead first? You can get large gains by doing that if you are on ext3.
blockdev --setra 2048 <device>

would give you a 1MB read-ahead. Also, consider XFS and its built-in defragmentation. I have found that a longer lived postgres DB will get extreme
file fragmentation over time and sequential scans end up mostly random. On-line file defrag helps tremendously.

> It maybe so, but slow sequential scan is still the largest single
> performance problem of PostgreSQL. The frequency with which that topic
> appears on the mailing lists should serve as a good evidence for that. I
> did my best to prove my case.

I'm not sure its all the I/O however. It seems that Postgres uses a lot more CPU than other DB's to crack open a tuple and inspect it. Testing on unindexed tables with count(*) I can get between 200MB and 800MB per second off disk max with full cpu utilization (depending on the average tuple size and contents). This is on a disk array that can do 1200MB/sec. It always feels dissapointing to not be able to max out the disk throughput on the simplest possible query.

> Again, requiring "hard numbers" when
> using the database which doesn't allow tweaking of the I/O size is self
> defeating proposition. The other databases, like DB2 and Oracle both
> allow tweaking of that aspect of its operation, Oracle even on the per
> session basis. If you still claim that it wouldn't make the difference,
> the onus to prove it is on you.
>
> --
> Mladen Gogala
> Sr. Oracle DBA
> 1500 Broadway
> New York, NY 10036
> (212) 329-5251
> www.vmsinfo.com
>
>
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