Simplify and speed up Postgres query for primary_keys

primary_keys(table) needs to query various metadata tables in Postgres to
determine the primary key for the table. Previously, it did so using a
complex common table expression against pg_constraint and pg_attribute.

This patch simplifies the query by joining pg_index against pg_attribute
instead of going through pg_constraint. This avoids an expensive unnest,
window function query, and common table expression.

EXPLAINing these queries in Postgres against a database with a single
table with a composite primary key shows a 66% reduction in the plan and
execute latencies. This is significant during application startup time,
especially against very large schemas, where these queries would be even
slower and more numerous.

Closes #27949

1 files changed, 8 insertions, 10 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index a4b1723fc7..bd8aab526e 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -426,16 +426,14 @@ module ActiveRecord
 
         def primary_keys(table_name) # :nodoc:
           select_values(<<-SQL.strip_heredoc, "SCHEMA")
-            WITH pk_constraint AS (
-              SELECT conrelid, unnest(conkey) AS connum FROM pg_constraint
-              WHERE contype = 'p'
-                AND conrelid = #{quote(quote_table_name(table_name))}::regclass
-            ), cons AS (
-              SELECT conrelid, connum, row_number() OVER() AS rownum FROM pk_constraint
-            )
-            SELECT attr.attname FROM pg_attribute attr
-            INNER JOIN cons ON attr.attrelid = cons.conrelid AND attr.attnum = cons.connum
-            ORDER BY cons.rownum
+                SELECT a.attname
+                  FROM pg_index i
+            CROSS JOIN unnest(i.indkey) as k
+                  JOIN pg_attribute a
+                    ON a.attrelid = i.indrelid
+                   AND a.attnum = k
+                 WHERE i.indrelid = #{quote(quote_table_name(table_name))}::regclass
+                   AND i.indisprimary
           SQL
         end