What is DEALLOCATE pdo_stmt_00000001 in pg_stats_activity "query"? In my return from pg_stat_activity, there are many processes in the "query" column that have the value "DEALLOCATE pdo_stmt_00000001".

What does it mean?
The state of these connections is probably (and hopefully) idle.

That means that the last thing that happened on these connections was to deallocate a prepared statement.

That is kind of funny, because one of the key advantages of prepared statements is to exist for a longer time and be reused. Maybe the application uses a connection pool and wants to &ldquo;clean&rdquo; the connection before returning it to the pool.

Anyway, there is nothing to worry about.