Does it matter which columns I select?
No, it doesn’t matter. Even if SELECT 1 FROM table WHERE ... FOR UPDATE is used, the query locks all rows that meet where conditions.
If the query retrieves rows from a join, and we don’t want to lock rows from all tables involved in the join, but only rows from specific tables, a SELECT ... FOR UPDATE OF list-of-tablenames syntax can be usefull:
http://www.postgresql.org/docs/9.0/static/sql-select.html#SQL-FOR-UPDATE-SHARE
I can’t do a select in a function without saving the data somewhere, so I save to a dummy variable. This seems hacky; is it the right way to do things?
In Pl/PgSql use a PERFORM command to discard query result:
http://www.postgresql.org/docs/9.2/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-SQL-NORESULT
Instead of:
SELECT 1 INTO dummy FROM my_table WHERE userid=v_1 LIMIT 1 FOR UPDATE;
use:
PERFORM 1 FROM my_table WHERE userid=v_1 LIMIT 1 FOR UPDATE;