How to set value of composite variable field using dynamic SQL

Faster with hstore

Since Postgres 9.0, with the additional module hstore installed in your database there is a very simple and fast solution with the #= operator that …

replace[s] fields in record with matching values from hstore.

To install the module:

CREATE EXTENSION hstore;

Examples:

SELECT my_record #= '"field"=>"value"'::hstore;  -- with string literal
SELECT my_record #= hstore(field, value);        -- with values

Values have to be cast to text and back, obviously.

Example plpgsql functions with more details:

• Endless loop in trigger function
• Assign to NEW by key in a Postgres trigger

Now works with json / jsonb, too!

There are similar solutions with json (pg 9.3+) or jsonb (pg 9.4+)

SELECT json_populate_record (my_record, json_build_object('key', 'new-value');

The functionality was undocumented, but it’s official since Postgres 13. The manual:

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

So you can take any existing row and fill arbitrary fields (overwriting what’s in it).

Major advantages of json vs hstore:

• works with stock Postgres so you don’t need an additional module.
• also works for nested array and composite types.

Minor disadvantage: a bit slower.

See @Geir’s added answer for details.

Without hstore and json

If you are on an older version or cannot install the additional module hstore or cannot assume it’s installed, here is an improved version of what I posted previously. Still slower than the hstore operator, though:

CREATE OR REPLACE FUNCTION f_setfield(INOUT _comp_val anyelement
                                          , _field text, _val text)
  RETURNS anyelement
  LANGUAGE plpgsql STABLE AS
$func$
BEGIN

EXECUTE 'SELECT ' || array_to_string(ARRAY(
      SELECT CASE WHEN attname = _field
                THEN '$2'
                ELSE '($1).' || quote_ident(attname)
             END AS fld
      FROM   pg_catalog.pg_attribute
      WHERE  attrelid = pg_typeof(_comp_val)::text::regclass
      AND    attnum > 0
      AND    attisdropped = FALSE
      ORDER  BY attnum
      ), ',')
USING  _comp_val, _val
INTO   _comp_val;

END
$func$;

Call:

CREATE TEMP TABLE t( a int, b text);  -- Composite type for testing
SELECT f_setfield(NULL::t, 'a', '1');

Notes

• An explicit cast of the value _val to the target data type is not necessary, a string literal in the dynamic query would be coerced automatically, obviating the subquery on pg_type. But I took it one step further:

• Replace quote_literal(_val) with direct value insertion via the USING clause. Saves one function call and two casts, and is safer anyway. text is coerced to the target type automatically in modern PostgreSQL. (Did not test with versions before 9.1.)

• array_to_string(ARRAY()) is faster than string_agg().

• No variables needed, no DECLARE. Fewer assignments.

• No subquery in the dynamic SQL. ($1).field is faster.

• pg_typeof(_comp_val)::text::regclass
  does the same as
  (SELECT typrelid FROM pg_catalog.pg_type WHERE oid = pg_typeof($1)::oid)
  for valid composite types, just faster.
  This last modification is built on the assumption that pg_type.typname is always identical to the associated pg_class.relname for registered composite types, and the double cast can replace the subquery. I ran this test in a big database to verify, and it came up empty as expected:

    SELECT *
    FROM   pg_catalog.pg_type t
    JOIN   pg_namespace  n ON n.oid = t.typnamespace
    WHERE  t.typrelid > 0  -- exclude non-composite types
    AND    t.typrelid IS DISTINCT FROM
          (quote_ident(n.nspname ) || '.' || quote_ident(typname))::regclass

• The use of an INOUT parameter obviates the need for an explicit RETURN. This is just a notational shortcut. Pavel won’t like it, he prefers an explicit RETURN statement …

Everything put together this is twice as fast as the previous version.

---

Original (outdated) answer:

The result is a version that’s ~ 2.25 times faster. But I probably couldn’t have done it without building on Pavel’s second version.

In addition, this version avoids most of the casting to text and back by doing everything within a single query, so it should be much less error prone.
Tested with PostgreSQL 9.0 and 9.1.

CREATE FUNCTION f_setfield(_comp_val anyelement, _field text, _val text)
  RETURNS anyelement
  LANGUAGE plpgsql STABLE AS
$func$
DECLARE
   _list text;
BEGIN
_list := (
   SELECT string_agg(x.fld, ',')
   FROM  (
      SELECT CASE WHEN a.attname = $2
              THEN quote_literal($3) || '::'|| (SELECT quote_ident(typname)
                                                FROM   pg_catalog.pg_type
                                                WHERE  oid = a.atttypid)
              ELSE quote_ident(a.attname)
             END AS fld
      FROM   pg_catalog.pg_attribute a 
      WHERE  a.attrelid = (SELECT typrelid
                           FROM   pg_catalog.pg_type
                           WHERE  oid = pg_typeof($1)::oid)
      AND    a.attnum > 0
      AND    a.attisdropped = false
      ORDER  BY a.attnum
      ) x
   );

EXECUTE 'SELECT ' || _list || ' FROM  (SELECT $1.*) x'
USING  $1
INTO   $1;

RETURN $1;
END
$func$;