Re: proposal: row_to_array function

Hi

2015-06-22 5:18 GMT+02:00 Craig Ringer <craig(at)2ndquadrant(dot)com>:

> On 2 April 2015 at 01:59, Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:
> > On Sun, Mar 29, 2015 at 1:27 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> >>> here is rebased patch.
> >>> It contains both patches - row_to_array function and foreach array
> support.
> >>
> >> While I don't have a problem with hstore_to_array, I don't think that
> >> row_to_array is a very good idea; it's basically encouraging people to
> >> throw away SQL datatypes altogether and imagine that everything is text.
> >> They've already bought into that concept if they are using hstore or
> >> json, so smashing elements of those containers to text is not a problem.
> >> But that doesn't make this version a good thing.
> >>
> >> (In any case, those who insist can get there through row_to_json, no?)
> >
> > You have a point. What does attached do that to_json does not do
> > besides completely discard type information? Our json api is pretty
> > rich and getting richer. For better or ill, we dumped all json
> > support into the already stupendously bloated public namespace and so
> > it's always available.
>
>
> I can see plenty of utility for a function like Pavel speaks of, but
> I'd personally rather see it as a function that returns table (colname
> name, coltype regtype, coltypmod integer, coltextvalue text,
> colordinal integer) so it can carry more complete information and
> there's no need to worry about foreach(array). The main use of a
> function that includes text representations of the values would IMO be
> using it from plain SQL, rather than PL/PgSQL, when faced with
> anonymous records.
>
> I'd find it more useful to have lvalue-expressions for dynamic access
> to record fields and a function to get record metadata - field names,
> types and typmods. Some kind of "pg_get_record_info(record) returns
> table(fieldname text, fieldtype regtype, fieldtypmod integer)" and a
> PL/PgSQL lvalue-expression for record field access like
> "RECORD_FIELD(therecord, fieldname)". I would _certainly_ want to be
> able to get the type metadata without the values.
>
> That way you could interact natively with the fields in their true
> types, without forcing conversion into and out of 'text', which is a
> known performance pain-point with PL/PgSQL. (PL/PgSQL doesn't have a
> VARIANT type or support for using 'anyelement', which would be the
> other way to solve the type flattening problem IMO).
>
> Think:
>
> DECLARE
> myrow record;
> fi record;
> BEGIN
> EXECUTE user_supplied_dynamic_query INTO myrow;
> FOR fi IN
> SELECT fieldname, fieldtype, fieldtypmod
> FROM pg_get_record_info(myrow)
> LOOP
> IF fi.fieldtype == 'int4'::regtype THEN
> RECORD_FIELD(myrow, fi.fieldname) := RECORD_FIELD(myrow,
> fi.fieldname) + 1;
> END IF;
> END LOOP;
> END;
>

I am thinking so this is separate task, that should not be solved simply
too. I wrote a set functions for working with record (
https://github.com/okbob/pltoolbox/blob/master/record.c) But it doesn't
solve the basic issues:

1. speed - FOR IN SELECT FROM is more expensive then just unpacking row or
record
2. unclean game with creating more code path for any special type.

I have little bit different idea. FOR IN RECORD can change type of any
automatic variable in any iteration. Internally we can do more code paths -
so your code can be rewritten to

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.

>
> OK, so it's a stupid example - increment all int4 fields by one. It
> conveys the rough idea though - native use of the field types.
>
> Note that RECORD_FIELD is distinct from the existing support for
>
> EXECUTE format('SELECT $1.%I', fieldname) USING therecord;
>
> in that that approach doesn't work for all ways that a record can be
> produced, it's slow, it doesn't have a good way to enumerate field
> names, and there's no equivalent to write to the field. Current
> approaches for that are ghastly:
> http://stackoverflow.com/q/7711432/398670 .
>
>
>
>
>
>
> --
> Craig Ringer http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Training & Services
>