From f9c6cbd55c56de7994b74630acd141c324411719 Mon Sep 17 00:00:00 2001 From: Jordan Lewis Date: Thu, 19 Jan 2017 15:29:17 -0500 Subject: 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. --- .../active_record/connection_adapters/postgresql_adapter.rb | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) (limited to 'activerecord/lib') 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 -- cgit v1.2.3