Re: proposal: row_to_array function - Mailing list pgsql-hackers

On 6/23/15 9:45 AM, Pavel Stehule wrote:

2015-06-23 1:56 GMT+02:00 Jim Nasby <Jim.Nasby@bluetreble.com
<mailto:Jim.Nasby@bluetreble.com>>:

    On 6/22/15 2:46 AM, Pavel Stehule wrote:


        FOREACH key, val IN RECORD myrow
        LOOP
            IF pg_typeof(val) IN ('int4', 'double precision', 'numeric')
        THEN
              val := val + 1; -- these variables can be mutable
              -- or maybe in futore
             myrow[key] := val + 1;
            END IF;
        END LOOP;

        What is important - "val" is automatic variable, and it can has
        different type in any step.

        It is little bit strange, but impossible to solve, so we cannot to
        support row[var] as right value (without immutable casting). But
        we can
        do it with left value.


    Actually, you can (theoretically) solve it for the right value as
    well with if val is an actual type and you have operators on that
    type that know to search for a specific operator given the actual
    types that are involved. So if val is int4, val + 1 becomes int4 + int4.

    The problem I've run into with this is by the time you've added
    enough casts to make this workable you've probably created a
    situation where val + something is going to recurse back to itself.
    I've partially solved this in [1], and intend to finish it by
    calling back in via SPI to do the final resolution, the same way the
    RI triggers do.

    What would be a lot better is if we had better control over function
    and operator resolution.

    [1]
    https://github.com/decibel/variant/commit/2b99067744a405da8a325de1ebabd213106f794f#diff-8aa2db4a577ee4201d6eb9041c2a457eR846


The solution of dynamic operators changes philosophy about 180° - and I
afraid about a performance.

Now if I am thinking about possibilities - probably it is solvable on
right side too. It needs to solve two steps:

1. parametrized record reference syntax - some like SELECT $1[$]
2. possibility to throw plan cache, if result has different type than is
expected in cache.

Well, the other option is we allow for cases where we don't know in advance what the type will be. That would handle this, JSON, variant, and possibly some other scenarios.

BTW, I think this relates to the desire to be able to do more OO-ish things in the database. Like "do X to all elements in this array". And to have actual classes, private members, real arrays of arrays. It seems like there's a bigger need here that's only being addressed piecemeal. :/

--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Data in Trouble? Get it in Treble! http://BlueTreble.com