On 05/03/12 04:06, Chris Angelico wrote:
On Mon, Mar 5, 2012 at 2:50 PM, David Johnston<polobo@xxxxxxxxx> wrote:
Any efficient, non-RegEx, alternative would require more context to evaluate than you provide. Mainly, would it be faster to have a separate field to store the parsed (at input) number and then query that field directly (even if it is a text field as well)? Basically cache the parse.
Caching's looking tempting, but I don't know if it'll be worth it
(these fields won't be searched-as-int very often compared to
search-as-string, and there's potentially a lot of such fields). All I
need out of it is the leading digits - I can strip them with trim(),
but I can't keep _only_ those digits.
The other possibility that may be of value is to write the function in
C instead of pl/pgsql, which will then actually call atoi() itself. Is
that going to be a better option?
Can you use to_number() here? It sounds like something along the lines
of cast(to_number('0' || field::varchar, '999999999.') as int) might
give the behaviour you're after, and a quick test seems to indicate that
it's about 4x faster than the original function:
postgres=# explain analyze select cast(to_number('0' ||
generate_series::varchar, '999999999.') as int) from
generate_series(1,1000000);
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Function Scan on generate_series (cost=0.00..22.50 rows=1000 width=4)
(actual time=137.720..1065.752 rows=1000000 loops=1)
Total runtime: 1144.993 ms
(2 rows)
postgres=# explain analyze select str2int(generate_series::varchar) from
generate_series(1,1000000);
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
Function Scan on generate_series (cost=0.00..265.00 rows=1000
width=4) (actual time=135.180..4022.408 rows=1000000 loops=1)
Total runtime: 4121.233 ms
(2 rows)
Hopefully there's a cleaner way of writing that without a long list of
9s in the format string, and if the field is nullable I'd guess you
probably need a coalesce(..., 0) around that as well.
cheers,
Tom
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