Split column into multiple rows in Postgres

In Postgres 9.3+ use a LATERAL join. Minimal form:

SELECT token, flag
FROM   tbl, unnest(string_to_array(subject, ' ')) token
WHERE  flag = 2;

The comma in the FROM list is (almost) equivalent to CROSS JOIN, LATERAL is automatically assumed for set-returning functions (SRF) in the FROM list. Why "almost"? See:

• "invalid reference to FROM-clause entry for table" in Postgres query

The alias "token" for the derived table is also assumed as column alias for a single anonymous column, and we assumed distinct column names across the query. Equivalent, more verbose and less error-prone:

SELECT s.token, t.flag
FROM   tbl t
CROSS  JOIN LATERAL unnest(string_to_array(subject, ' ')) AS s(token)
WHERE  t.flag = 2;

Or move the SRF to the SELECT list, which is allowed in Postgres (but not in standard SQL), to the same effect:

SELECT unnest(string_to_array(subject, ' ')) AS token, flag
FROM   tbl
WHERE  flag = 2;

The last one seems acceptable since SRF in the SELECT list have been sanitized in Postgres 10. See:

• What is the expected behaviour for multiple set-returning functions in SELECT clause?

If unnest() does not return any rows (empty or NULL subject), the (implicit) join eliminates the row from the result. Use LEFT JOIN ... ON true to keep qualifying rows from tbl. See:

• What is the difference between LATERAL JOIN and a subquery in PostgreSQL?

We could also use regexp_split_to_table(), but that's typically slower because regular expressions cost a bit more. See:

• SQL select rows containing substring in text field
• PostgreSQL unnest() with element number

I think it's not necessary to use a join, just the unnest() function in conjunction with string_to_array() should do it:

SELECT unnest(string_to_array(subject, ' ')) as "token", flag FROM test;

token | flag                                                                                                   
-------+-------                                                                                                  
this   |     2                                                                                                   
is     |     2                                                                                                   
a      |     2                                                                                                   
test   |     2