PostgreSQL unnest() with element number

Postgres 9.4 or later

Use WITH ORDINALITY for set-returning functions:

When a function in the FROM clause is suffixed by WITH ORDINALITY, abigint column is appended to the output which starts from 1 andincrements by 1 for each row of the function's output. This is mostuseful in the case of set returning functions such as unnest().

In combination with the LATERAL feature in pg 9.3+, and according to this thread on pgsql-hackers, the above query can now be written as:

SELECT t.id, a.elem, a.nrFROM   tbl AS tLEFT   JOIN LATERAL unnest(string_to_array(t.elements, ','))                    WITH ORDINALITY AS a(elem, nr) ON TRUE;

LEFT JOIN ... ON TRUE preserves all rows in the left table, even if the table expression to the right returns no rows. If that's of no concern you can use this otherwise equivalent, less verbose form with an implicit CROSS JOIN LATERAL:

SELECT t.id, a.elem, a.nrFROM   tbl t, unnest(string_to_array(t.elements, ',')) WITH ORDINALITY a(elem, nr);

Or simpler if based off an actual array (arr being an array column):

SELECT t.id, a.elem, a.nrFROM   tbl t, unnest(t.arr) WITH ORDINALITY a(elem, nr);

Or even, with minimal syntax:

SELECT id, a, ordinalityFROM   tbl, unnest(arr) WITH ORDINALITY a;

a is automatically table and column alias. The default name of the added ordinality column is ordinality. But it's better (safer, cleaner) to add explicit column aliases and table-qualify columns.

Postgres 8.4 - 9.3

With row_number() OVER (PARTITION BY id ORDER BY elem) you get numbers according to the sort order, not the ordinal number of the original ordinal position in the string.

You can simply omit ORDER BY:

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

While this normally works and I have never seen it fail in simple queries, PostgreSQL asserts nothing concerning the order of rows without ORDER BY. It happens to work due to an implementation detail.

To guarantee ordinal numbers of elements in the blank-separated string:

SELECT id, arr[nr] AS elem, nrFROM  (   SELECT *, generate_subscripts(arr, 1) AS nr   FROM  (SELECT id, string_to_array(elements, ' ') AS arr FROM tbl) t   ) sub;

Or simpler if based off an actual array:

SELECT id, arr[nr] AS elem, nrFROM  (SELECT *, generate_subscripts(arr, 1) AS nr FROM tbl) t;

Related answer on dba.SE:

• How to preserve the original order of elements in an unnested array?

Postgres 8.1 - 8.4

None of these features are available, yet: RETURNS TABLE, generate_subscripts(), unnest(), array_length(). But this works:

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

Note in particular, that the array index can differ from ordinal positions of elements. Consider this demo with an extended function:

CREATE FUNCTION f_unnest_ord_idx(anyarray, OUT val anyelement, OUT ordinality int, OUT idx int)  RETURNS SETOF record  LANGUAGE sql IMMUTABLE AS'SELECT $1[i], i - array_lower($1,1) + 1, i FROM   generate_series(array_lower($1,1), array_upper($1,1)) i';SELECT id, arr, (rec).*FROM  (   SELECT *, f_unnest_ord_idx(arr) AS rec   FROM  (VALUES (1, '{a,b,c}'::text[])  --  short for: '[1:3]={a,b,c}'               , (2, '[5:7]={a,b,c}')               , (3, '[-9:-7]={a,b,c}')      ) t(id, arr)   ) sub; id |       arr       | val | ordinality | idx----+-----------------+-----+------------+-----  1 | {a,b,c}         | a   |          1 |   1  1 | {a,b,c}         | b   |          2 |   2  1 | {a,b,c}         | c   |          3 |   3  2 | [5:7]={a,b,c}   | a   |          1 |   5  2 | [5:7]={a,b,c}   | b   |          2 |   6  2 | [5:7]={a,b,c}   | c   |          3 |   7  3 | [-9:-7]={a,b,c} | a   |          1 |  -9  3 | [-9:-7]={a,b,c} | b   |          2 |  -8  3 | [-9:-7]={a,b,c} | c   |          3 |  -7

Compare:

• Normalize array subscripts for 1-dimensional array so they start with 1

Try:

select v.*, row_number() over (partition by id order by elem) rn from(select    id,    unnest(string_to_array(elements, ',')) AS elem from myTable) v

Use Subscript Generating Functions.
http://www.postgresql.org/docs/current/static/functions-srf.html#FUNCTIONS-SRF-SUBSCRIPTS

For example:

SELECT   id  , elements[i] AS elem  , i AS nrFROM  ( SELECT       id      , elements      , generate_subscripts(elements, 1) AS i    FROM      ( SELECT          id          , string_to_array(elements, ',') AS elements        FROM          myTable      ) AS foo  ) bar;

More simply:

SELECT  id  , unnest(elements) AS elem  , generate_subscripts(elements, 1) AS nrFROM  ( SELECT      id      , string_to_array(elements, ',') AS elements    FROM      myTable  ) AS foo;