Trigger function taking column names as parameters to modify the row

I am trying to write a trigger function in Postgres 9.4. Something like this (not working, yet):

CREATE FUNCTION set_point_from_coords(source _regclass, target _regclass) 
  RETURNS trigger AS
$func$
BEGIN
  NEW.target := ST_SetSRID(ST_Point(NEW.source[1], NEW.source[2]), 4326);
  RETURN NEW;
END;
$func$ LANGUAGE plpgsql

In this case, target is a column of type geometry and source is an array of decimals.

When a row is inserted with a coords array, I want to convert that to a point. The above would work if I just hard-coded column names, but I want to do it for different tables and different pairs of columns using the same function. And I don't have direct control over the INSERT itself.

Here's some of my experimentation: http://sqlfiddle.com/#!15/dddcd/1

Found this related blog post which I'm having a really hard time parsing.

I could live with this running after insert/update instead of before, if that makes it easier to code.

Problem

You had to pick the spot where all possible complications come together.

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SQL (or PL/pgSQL) does not allow to parameterize identifiers. You need dynamic SQL with EXECUTE for that.

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But the special PL/pgSQL variable NEW in trigger functions is not visible inside dynamic code executed with EXECUTE. You have to pass it in.

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Passing column names as arguments to the trigger function in CREATE TRIGGER further complicates the workflow.

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And it's not enough to make the target column dynamic, you want to fetch the source value from another dynamic column of the row.

Unless you know your way around the involved matters, rather try a simpler approach. Write a separate trigger function for each trigger and assign to the target column without dynamic SQL.
Solution

That said, it can be done with one line of code - and many lines of explanation. For the original example in the question, assuming this table definition:

CREATE TABLE tbl (
  tbl_id serial PRIMARY KEY
, geom geometry
, coords double precision[]
);

The first version of this answer used the additional module hstore. Since Postgres 13 an equivalent feature of json_populate_record() / jsonb_populate_record() is documented. See:

• How to set value of composite variable field using dynamic SQL

CREATE OR REPLACE FUNCTION trg_demo()
  RETURNS trigger
  LANGUAGE plpgsql AS 
$func$
BEGIN
   EXECUTE format('SELECT (json_populate_record($1, json_build_object(%L, ST_SetSRID(ST_Point($1.%2$I[1], $1.%2$I[2]), 4326)::text))).*'
                , TG_ARGV[0], TG_ARGV[1])  -- target (geom), source (coords)
   USING  NEW
   INTO   NEW;

   RETURN NEW;
END
$func$;

Trigger:

CREATE TRIGGER demo
BEFORE INSERT OR UPDATE ON tbl
FOR EACH ROW EXECUTE PROCEDURE trg_demo('geom', 'coords');

If you don't understand what's happening here, consider my advice above.
Explanation

I formatted the dynamic computation of the geometry bold to help you get a grip on this. Compare with the simple case below.

fiddle -- with point instead of geometry, as PostGIS is not installed.

Old sqlfiddle

Here is a simpler version, just assigning the bare text value (would require the target column geom to be type text). The simplified part in bold again:

EXECUTE format('SELECT (json_populate_record($1, json_build_object(%L, $1.%I))).*'
                , TG_ARGV[0], TG_ARGV[1])  -- target (geom), source (coords)
   USING  NEW
   INTO   NEW;

The core feature is this detail for json_populate_record(), documented since Postgres 13:

However, if base isn't NULL then the values it contains will be used for unmatched columns.

Both target and source are columns of the new row, which complicates matters. If the source value was a constant we could simply:

NEW := json_populate_record(NEW, json_build_object(TG_ARGV[0], 'POINT(123.0, 456.0)'));

Related:

• Assign to NEW by key in a Postgres trigger

But we need dynamic SQL to resolve the column name and fetch the source value.

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Use format() to concatenate the query string safely.

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TG_ARGV[0] and TG_ARGV[1] access the first two elements to the (0-based!) text array of arguments passed by CREATE TRIGGER.

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%I concatenates the argument passed to format() as identifier (safe against SQL injection).

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$1 references the value passed to EXECUTE in the USING clause.

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Casting to ::text after calculating the geometry because the intermittent json stores a string value.

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Decompose the row NEW for the assignment back, because PL/pgSQL assigns rows as target column-by-column.

StackExchange Database Administrators Q#127787, answer score: 19