Re: Speeding up loops in pl/pgsql function

On Thu, May 26, 2011 at 8:11 AM, Merlin Moncure <mmoncure@gmail.com> wrote:
> On Wed, May 25, 2011 at 9:20 PM, Merlin Moncure <mmoncure@gmail.com> wrote:
>> On Wed, May 25, 2011 at 8:03 PM, Steve Crawford
>> <scrawford@pinpointresearch.com> wrote:
>>> On 05/25/2011 11:45 AM, Reuven M. Lerner wrote:
>>>>
>>>> Hi, Alex.  You wrote:
>>>>>
>>>>> Have you tried something like:
>>>>> SELECT  encode(regexp_replace('141142143', '(\d{3})', '\\\1',
>>>>> 'g')::bytea, 'escape');
>>>>
>>>> Hmm, forgot about regexp_replace.  It might do the trick, but without a
>>>> full-blown eval that I can run on the replacement side, it'll be a bit more
>>>> challenging.  But that's a good direction to consider, for sure.
>>>
>>> The function given didn't work exactly as written for me but it is on the
>>> right track. See if this works for you (input validation is left as an
>>> exercise for the reader...:)):
>>>
>>> create or replace function octal_string_to_text(someoctal text) returns text
>>> as $$
>>> declare
>>>    binstring text;
>>> begin
>>>    execute 'select E''' || regexp_replace($1, E'(\\d{3})', E'\\\\\\1', 'g')
>>> || '''' into binstring;
>>> return binstring;
>>> end
>>> $$ language plpgsql;
>>
>> four points (minor suggestions btw):
>> 1. if you are dealing with strings that have backslashes in them,
>> don't escape, but dollar quote.  Also try not to use dollar parameter
>> notation if you can help it:
>> ($1, E'(\\d{3})', E'\\\\\\1', 'g') -> (someoctal , $q$(\d{3})$q$,
>> $q$\\\1$q$, 'g')
>>
>> this is particularly true with feeding strings to regex: that way you
>> can use the same string pg as in various validators.
>>
>> 2. there is no need for execute here.
>> execute 'select E''' || regexp_replace($1, E'(\\d{3})', E'\\\\\\1', 'g')
>> becomes:
>> binstring := 'E''' || regexp_replace($1, $q$(\d{3})$q$, $q$\\\1$q$,
>> 'g')  /* I *think* I got this right */
>>
>> 3. if your function does not scribble on tables and has no or is not
>> influenced by any side effects, mark it as IMMUTABLE. always.
>> $$ language plpgsql IMMUTABLE;
>>
>> 4. since all we are doing is generating a variable, prefer sql
>> function vs plpgsql. this is particularly true in pre 8.4 postgres
>> (IIRC) where you can call the function much more flexibly (select
>> func(); vs select * from func();) if that's the case.  Putting it all
>> together,
>>
>> create or replace function octal_string_to_text(someoctal text)
>> returns text as $$
>>   SELECT  'E''' || regexp_replace($1, $q$(\d{3})$q$, $q$\\\1$q$, 'g');
>> $$ sql immutable;
>>
>> Note I didn't actually check to see what your regex is donig (I'm
>> assuming it's correct)...
>
> hm, I slept on this and had the vague unsettling feeling I had said
> something stupid -- and I did.  Double +1 to you for being cleverer
> than me -- you are using 'execute' to eval the string back in to the
> string.  Only plpgsql can do that, so point 4 is also moot.  Still,
> the above points hold in principle, so if a way could be figured out
> to do this without execute, that would be nice.

got it:
select decode(regexp_replace('141142143', '([0-9][0-9][0-9])',
$q$\\\1$q$ , 'g'), 'escape');
 decode
--------
 abc
(1 row)

merlin