Postgres FOR LOOP

Procedural elements like loops are not part of the SQL language and can only be used inside the body of a procedural language function, procedure (Postgres 11 or later) or a DO statement, where such additional elements are defined by the respective procedural language. The default is PL/pgSQL, but there are others.

Example with plpgsql:

DO$do$BEGIN    FOR i IN 1..25 LOOP      INSERT INTO playtime.meta_random_sample         (col_i, col_id)                       -- declare target columns!      SELECT  i,     id      FROM   tbl      ORDER  BY random()      LIMIT  15000;   END LOOP;END$do$;

For many tasks that can be solved with a loop, there is a shorter and faster set-based solution around the corner. Pure SQL equivalent for your example:

INSERT INTO playtime.meta_random_sample (col_i, col_id)SELECT t.*FROM   generate_series(1,25) iCROSS  JOIN LATERAL (   SELECT i, id   FROM   tbl   ORDER  BY random()   LIMIT  15000   ) t;

About generate_series():

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

About optimizing performance of random selections:

• Best way to select random rows PostgreSQL

Below is example you can use:

create temp table test2 (  id1  numeric,  id2  numeric,  id3  numeric,  id4  numeric,  id5  numeric,  id6  numeric,  id7  numeric,  id8  numeric,  id9  numeric,  id10 numeric) with (oids = false);do$do$declare     i int;beginfor  i in 1..100000loop    insert into test2  values (random(), i * random(), i / random(), i + random(), i * random(), i / random(), i + random(), i * random(), i / random(), i + random());end loop;end;$do$;

I just ran into this question and, while it is old, I figured I'd add an answer for the archives. The OP asked about for loops, but their goal was to gather a random sample of rows from the table. For that task, Postgres 9.5+ offers the TABLESAMPLE clause on WHERE. Here's a good rundown:

https://www.2ndquadrant.com/en/blog/tablesample-in-postgresql-9-5-2/

I tend to use Bernoulli as it's row-based rather than page-based, but the original question is about a specific row count. For that, there's a built-in extension:

https://www.postgresql.org/docs/current/tsm-system-rows.html

CREATE EXTENSION tsm_system_rows;

Then you can grab whatever number of rows you want:

select * from playtime tablesample system_rows (15);