Gregory Stark wrote:
> "Bruce Momjian" <bruce@momjian.us> writes:
>
> >> No, we did substring() too :)
> >
> > Uh, I looked at text_substring(), and while there is an optimization to
> > do character counting for encoding length == 1, it is still accessing
> > the data.
>
> Sure but it'll only access the first chunk. There are two chunks in your test.
> It might be interesting to run tests accessing 0 (length()), 1 (substr()), and
> 2 chunks (hashtext()).
>
> Or if you're concerned with the cpu cost of hashtext you could calculate the
> precise two bytes you need to access with substr to force it to load both
> chunks. But I think the real cost of unnecessary toasting is the random disk
> i/o so the cpu cost is of secondary interest.
OK, will run a test with hashtext(). What I am seeing now is a 10-20x
slowdown to access the TOAST data, and a 0-1x speedup for accessing the
non-TOAST data when the rows are long:
http://momjian.us/expire/TOAST/
-- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB
http://www.enterprisedb.com
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