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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
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jordanlewis authored and jeremy committed Feb 8, 2017
1 parent 2b4d145 commit d6529af2954a67bd57fda45286fa9cfd0ff6b5ac
@@ -426,16 +426,14 @@ def pk_and_sequence_for(table) #:nodoc:
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
@@ -298,6 +298,10 @@ def setup
t.string :region
t.integer :code
@connection.create_table(:barcodes_reverse, primary_key: ["code", "region"], force: true) do |t|
t.string :region
t.integer :code
def teardown
@@ -308,6 +312,11 @@ def test_composite_primary_key
assert_equal ["region", "code"], @connection.primary_keys("barcodes")
def test_composite_primary_key_out_of_order
skip if current_adapter?(:SQLite3Adapter)
assert_equal ["region", "code"], @connection.primary_keys("barcodes")
def test_primary_key_issues_warning
model = do
def self.table_name

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