improve change_column_null

add a temporary check constraint first because it uses a nicer lock
for the long period of time. doing so means we don't need to bother
with pre-warming anymore

lib/active_record/pg_extensions/pessimistic_migrations.rb

@@ -6,20 +6,36 @@ module PGExtensions
     # to executing, in order to reduce the amount of time the actual DDL takes to
     # execute (and thus how long it needs the lock)
     module PessimisticMigrations
-      # does a query first to warm the db cache, to make the actual constraint adding fast
+      # adds a temporary check constraint to reduce locking when changing to NOT NULL, and we're not in a transaction
       def change_column_null(table, column, nullness, default = nil)
-        # no point in pre-warming the cache to avoid locking if we're already in a transaction
+        # no point in doing extra work to avoid locking if we're already in a transaction
         return super if nullness != false || open_transactions.positive?
+        return if columns(table).find { |c| c.name == column.to_s }&.null == false
+
+        temp_constraint_name = "chk_rails_#{table}_#{column}_not_null"
+        scope = quoted_scope(temp_constraint_name)
+        # check for temp constraint
+        valid = select_value(<<~SQL, "SCHEMA")
+          SELECT convalidated FROM pg_constraint INNER JOIN pg_namespace ON pg_namespace.oid=connamespace WHERE conname=#{scope[:name]} AND nspname=#{scope[:schema]}
+        SQL
+        if valid.nil?
+          add_check_constraint(table,
+                               "#{quote_column_name(column)} IS NOT NULL",
+                               name: temp_constraint_name,
+                               validate: false)
+        end
+        begin
+          validate_constraint(table, temp_constraint_name)
+        rescue ActiveRecord::StatementInvalid => e
+          raise ActiveRecord::NotNullViolation.new(sql: e.sql, binds: e.binds) if e.cause.is_a?(PG::CheckViolation)
+
+          raise
+        end
 
         transaction do
-          # make sure the query ignores indexes, because the actual ALTER TABLE will also ignore
-          # indexes
-          execute("SET LOCAL enable_indexscan=off")
-          execute("SET LOCAL enable_bitmapscan=off")
-          execute("SELECT COUNT(*) FROM #{quote_table_name(table)} WHERE #{quote_column_name(column)} IS NULL")
-          raise ActiveRecord::Rollback
+          super
+          remove_check_constraint(table, name: temp_constraint_name)
         end
-        super
       end
 
       # several improvements:

spec/pessimistic_migrations_spec.rb

@@ -6,32 +6,61 @@
   end
 
   describe "#change_column_null" do
+    # Rails 6.1 doesn't quote the constraint name when adding a check constraint??
+    def quote_constraint_name(name)
+      Rails.version >= "6.1" ? name : connection.quote_column_name(name)
+    end
+
     it "does nothing extra when changing a column to nullable" do
       connection.change_column_null(:table, :column, true)
       expect(connection.executed_statements).to eq ['ALTER TABLE "table" ALTER COLUMN "column" DROP NOT NULL']
     end
 
-    it "pre-warms the cache" do
+    it "does nothing if we're in a transaction" do
+      connection.transaction do
+        connection.change_column_null(:table, :column, true)
+      end
+      expect(connection.executed_statements).to eq ["BEGIN",
+                                                    'ALTER TABLE "table" ALTER COLUMN "column" DROP NOT NULL',
+                                                    "COMMIT"]
+    end
+
+    it "skips entirely if the column is already NOT NULL" do
+      expect(connection).to receive(:columns).with(:table).and_return([double(name: "column", null: false)])
       connection.change_column_null(:table, :column, false)
-      expect(connection.executed_statements).to eq(
-        ["BEGIN",
-         "SET LOCAL enable_indexscan=off",
-         "SET LOCAL enable_bitmapscan=off",
-         'SELECT COUNT(*) FROM "table" WHERE "column" IS NULL',
-         "ROLLBACK",
-         'ALTER TABLE "table" ALTER COLUMN "column" SET NOT NULL']
-      )
+      expect(connection.executed_statements).to eq([])
     end
 
-    it "does nothing extra if a transaction is already active" do
-      connection.transaction do
-        connection.change_column_null(:table, :column, false)
-      end
-      expect(connection.executed_statements).to eq(
-        ["BEGIN",
-         'ALTER TABLE "table" ALTER COLUMN "column" SET NOT NULL',
-         "COMMIT"]
-      )
+    it "adds and removes a check constraint" do
+      expect(connection).to receive(:columns).and_return([])
+      allow(connection).to receive(:check_constraint_for!).and_return(double(name: "chk_rails_table_column_not_null"))
+      connection.change_column_null(:table, :column, false)
+
+      expect(connection.executed_statements).to eq [
+        "SELECT convalidated FROM pg_constraint INNER JOIN pg_namespace ON pg_namespace.oid=connamespace WHERE conname='chk_rails_table_column_not_null' AND nspname=ANY (current_schemas(false))\n", # rubocop:disable Layout/LineLength
+        %{ALTER TABLE "table" ADD CONSTRAINT #{quote_constraint_name('chk_rails_table_column_not_null')} CHECK ("column" IS NOT NULL) NOT VALID}, # rubocop:disable Layout/LineLength
+        'ALTER TABLE "table" VALIDATE CONSTRAINT "chk_rails_table_column_not_null"',
+        "BEGIN",
+        'ALTER TABLE "table" ALTER COLUMN "column" SET NOT NULL',
+        'ALTER TABLE "table" DROP CONSTRAINT "chk_rails_table_column_not_null"',
+        "COMMIT"
+      ]
+    end
+
+    it "verifies an existing check constraint" do
+      expect(connection).to receive(:columns).and_return([])
+      allow(connection).to receive(:check_constraint_for!).and_return(double(name: "chk_rails_table_column_not_null"))
+      expect(connection).to receive(:select_value).and_return(false)
+      connection.change_column_null(:table, :column, false)
+
+      expect(connection.executed_statements).to eq [
+        # stubbed out <check constraint valid>
+        'ALTER TABLE "table" VALIDATE CONSTRAINT "chk_rails_table_column_not_null"',
+        "BEGIN",
+        'ALTER TABLE "table" ALTER COLUMN "column" SET NOT NULL',
+        'ALTER TABLE "table" DROP CONSTRAINT "chk_rails_table_column_not_null"',
+        "COMMIT"
+      ]
     end
   end