Re: EXTERNAL storage and substring on long strings

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.

Any other ideas out there?

Joe