Simplify Postgres query for column_definitions()

column_definitions() needs to fetch the collation for every column, if
present. Previously, it did so using a correlated subquery - a subquery
that references results from the outer scope.

This patch updates the query to remove the subquery in favor of a
simpler and more efficient JOIN clause.

Running the two queries through EXPLAIN against Postgres additionally
shows that the original form with a correlated subquery requires a
Nested Loop Left Join, while the new form with a simple JOIN can use a
more efficient Merge Left Join.

0 files changed, 0 insertions, 0 deletions