Rowley: What’s new in the Postgres 16 query planner / optimizer

David Rowley looksdeeply into the improvements coming to the query planner inPostgreSQL16.

For a long time now, PostgreSQL has been able to remove a LEFT JOINwhere no column from the left joined table was required in thequery and the join could not possibly duplicate any rows.

However, in versions prior to PostgreSQL 16, there was no supportfor left join removals on partitioned tables. Why? Because theproofs that the planner uses to determine if there's anypossibility any inner-side row could duplicate any outer-side rowwere not present for partitioned tables.

The PostgreSQL 16 query planner now allows the LEFT JOIN removaloptimization with partitioned tables.