Using temp table in PL/pgSQL procedure for cleaning tables

Question

I\'m trying to delete all data related to a user id from a game database.

There is a table holding all games (each played by 3 players):

# select * f         


        

Answer 1

Besides explicitly creating a temporary table and then inserting into it, there is another, simpler, right way to do it: CREATE TEMP TABLE AS as recommended in the docs:

This command is functionally similar to SELECT INTO, but it is preferred since it is less likely to be confused with other uses of the SELECT INTO syntax. Furthermore, CREATE TABLE AS offers a superset of the functionality offered by SELECT INTO.

For Postgres 9.1 or later see below.

It would also be more efficient to use DELETE .. USING .. instead of a sub-select.
And yes, if you are not planning to keep using the temp table (in the same session) after the transaction is committed, add ON COMMIT DROP.

Putting it all together, your function could look like this:

CREATE OR REPLACE FUNCTION pref_delete_user(_id varchar)
  RETURNS void AS
$func$
BEGIN    
   CREATE TEMP TABLE tmp_gids ON COMMIT DROP AS
   SELECT gid FROM pref_scores WHERE id = _id;

   DELETE FROM pref_scores p
   USING  tmp_gids t
   WHERE  p.gid = t.gid;

   DELETE FROM pref_games p
   USING  tmp_gids t
   WHERE  p.gid = t.gid;

   -- more deletes ...    
END
$func$ LANGUAGE plpgsql;

Data modifying CTE

In modern day Postgres the above only makes sense for complicated operations where you need an actual temporary table to work with - for instance to create an index on it before you proceed.

In Postgres 9.1 or later use data-modifying CTEs for simple cases like the one at hand:

   WITH gids AS (SELECT gid FROM pref_scores WHERE id = _id)
      , d1   AS (  
      DELETE FROM pref_scores p
      USING  gids t
      WHERE  p.gid = t.gid
      (
      -- more work using gids?
   DELETE FROM pref_games p
   USING  gids t
   WHERE  p.gid = t.gid;

Answer 2

You can try

EXECUTE 'create temp table temp_gids AS select from pref_scores where id=$1'
    USING _id;

Answer 3

You could create the temporary table and then do the usual INSERT ... SELECT as separate operations:

create temporary table temp_gids (gid int not null) on commit drop;
insert into temp_gids (gid) select gid from pref_scores where id = _id;

There's also a LIKE option to CREATE TABLE if you want to duplicate a table's structure:

LIKE parent_table [ like_option ... ]
The LIKE clause specifies a table from which the new table automatically copies all column names, their data types, and their not-null constraints.

But I think you just need a temporary table to hold some IDs so that's probably overkill.

SELECT INTO works as you expect outside a procedure:

[...] this form of SELECT INTO is not available in ECPG or PL/pgSQL, because they interpret the INTO clause differently.

SELECT INTO is used to store the result of a SELECT in a local variable inside a PostgreSQL procedure:

The result of a SQL command yielding a single row (possibly of multiple columns) can be assigned to a record variable, row-type variable, or list of scalar variables. This is done by writing the base SQL command and adding an INTO clause.