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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.
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