Re: EXTERNAL storage and substring on long strings

On Thu, 2003-07-31 at 15:31, Joe Conway wrote:
> Scott Cain wrote:
> >  Index Scan using feature_pkey on feature  (cost=0.00..3.01 rows=1
> > width=153) (actual time=954.13..954.14 rows=1 loops=1)
> >    Index Cond: (feature_id = 1)
> >  Total runtime: 954.26 msec
> > (3 rows)
> >
> > Whoa!  That's not what I expected, the time to do the query got more
> > that twice as long.  So I think, maybe it was just an unlucky section,
> > and overall performance will be much better.  So I write a perl script
> > to do substring queries over all of my chromosomes at various positions
> > and lengths (20,000 queries total).  For comparison, I also ran the same
> > script, extracting the chromosomes via sql and doing the substring in
> > perl.  Here's what happened:
>
> Hmmm, what happens if you compare with a shorter substring, e.g.:
>
> explain analyze select substring(residues from 1000000 for 2000)
> from feature where feature_id=1;
>
> I'm just guessing, but it might be that the extra I/O time to read 20K
> of uncompressed text versus the smaller compressed text is enough to
> swamp the time saved from not needing to uncompress.

Are you asking, "Can his CPU decompress faster than his disks can
read?"

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| Ron Johnson, Jr.        Home: ron.l.johnson@cox.net             |
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