How to Efficiently Unnest Comma-Separated Strings in PostgreSQL?

PostgreSQL 14 and above

For comma separated strings, use the string_to_table() function instead of unnest(string_to_array()):

SELECT t.id, a.elem, a.nr
FROM tbl t
LEFT JOIN LATERAL string_to_table(t.elements, ',') WITH ORDINALITY AS a(elem, nr) ON true;

PostgreSQL 9.4 and above

Append WITH ORDINALITY for functions that return collections:

SELECT t.id, a.elem, a.nr
FROM tbl AS t
LEFT JOIN LATERAL unnest(string_to_array(t.elements, ',')) WITH ORDINALITY AS a(elem, nr) ON true;

PostgreSQL 8.4 - 9.3 versions

Use row_number() OVER (PARTITION BY id) (without ORDER BY) to get the serial number:

SELECT *, row_number() OVER (PARTITION by id) AS nr
FROM (SELECT id, regexp_split_to_table(elements, ',') AS elem FROM tbl) t;

PostgreSQL 8.1 - 8.4 versions

Implement custom function f_unnest_ord() for splitting elements with serial numbers:

CREATE FUNCTION f_unnest_ord(anyarray, OUT val anyelement, OUT ordinality integer)
RETURNS SETOF record
LANGUAGE sql IMMUTABLE AS
'SELECT [i], i - array_lower(,1) + 1
FROM generate_series(array_lower(,1), array_upper(,1)) i';

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