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.

1 files changed, 5 insertions, 5 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index 0ebd907cc0..315d70c33f 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -759,11 +759,11 @@ module ActiveRecord
           query(<<-end_sql, "SCHEMA")
               SELECT a.attname, format_type(a.atttypid, a.atttypmod),
                      pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod,
-             (SELECT c.collname FROM pg_collation c, pg_type t
-               WHERE c.oid = a.attcollation AND t.oid = a.atttypid AND a.attcollation <> t.typcollation),
-                     col_description(a.attrelid, a.attnum) AS comment
-                FROM pg_attribute a LEFT JOIN pg_attrdef d
-                  ON a.attrelid = d.adrelid AND a.attnum = d.adnum
+                     c.collname, col_description(a.attrelid, a.attnum) AS comment
+                FROM pg_attribute a
+                LEFT JOIN pg_attrdef d ON a.attrelid = d.adrelid AND a.attnum = d.adnum
+                LEFT JOIN pg_type t ON a.atttypid = t.oid
+                LEFT JOIN pg_collation c ON a.attcollation = c.oid AND a.attcollation <> t.typcollation
                WHERE a.attrelid = #{quote(quote_table_name(table_name))}::regclass
                  AND a.attnum > 0 AND NOT a.attisdropped
                ORDER BY a.attnum