Placing query output into psql variables

It is also possible to store some values produced by a query into variables, for instance to reuse them later in other queries.

In this recipe we demonstrate this approach with a concrete example.

Getting ready

In recipe Controlling automatic database maintenance of Chapter 9, Regular Maintenance, we describe VACUUM, showing that it runs regularly on each table based on the number of rows that might need vacuuming ("dead rows"). VACUUM will run if that number exceeds a given threshold, which by default is just above 20% of the row count.

In this recipe we create a script that picks the table with the largest number of dead rows, and runs VACUUM on it.

How to do it…

The script is as follows:

SELECT schemaname
, relname
, n_dead_tup
, n_live_tup
FROM pg_stat_user_tables
ORDER BY n_dead_tup DESC
LIMIT 1
\gset
\qecho Running VACUUM on table :"relname" in schema :"schemaname"
\qecho Rows before: :n_dead_tup dead, :n_live_tup live
VACUUM ANALYZE :schemaname.:relname...