Split CALC_FOUND_ROW query into seperate count query for better performance (#35468)

Co-authored-by: Néstor Soriano <konamiman@konamiman.com>

plugins/woocommerce/changelog/fix-35464

@@ -0,0 +1,4 @@
+Significance: patch
+Type: performance
+
+Split CALC_FOUND_ROW query into seperate count query for better performance.

plugins/woocommerce/src/Internal/DataStores/Orders/OrdersTableQuery.php

@@ -115,6 +115,13 @@ class OrdersTableQuery {
 	 */
 	private $sql = '';
 
+	/**
+	 * Final SQL query to count results after processing of args.
+	 *
+	 * @var string
+	 */
+	private $count_sql = '';
+
 	/**
 	 * The number of pages (when pagination is enabled).
 	 *
@@ -586,11 +593,7 @@ class OrdersTableQuery {
 		$fields       = $this->fields;
 
 		// SQL_CALC_FOUND_ROWS.
-		if ( ( ! $this->arg_isset( 'no_found_rows' ) || ! $this->args['no_found_rows'] ) && $this->limits ) {
-			$found_rows = 'SQL_CALC_FOUND_ROWS';
-		} else {
 		$found_rows = '';
-		}
 
 		// JOIN.
 		$join = implode( ' ', array_unique( array_filter( array_map( 'trim', $this->join ) ) ) );
@@ -617,6 +620,23 @@ class OrdersTableQuery {
 		$groupby = $this->groupby ? 'GROUP BY ' . implode( ', ', (array) $this->groupby ) : '';
 
 		$this->sql = "SELECT $found_rows DISTINCT $fields FROM $orders_table $join WHERE $where $groupby $orderby $limits";
+		$this->build_count_query( $fields, $join, $where, $groupby );
+	}
+
+	/**
+	 * Build SQL query for counting total number of results.
+	 *
+	 * @param string $fields Prepared fields for SELECT clause.
+	 * @param string $join Prepared JOIN clause.
+	 * @param string $where Prepared WHERE clause.
+	 * @param string $groupby Prepared GROUP BY clause.
+	 */
+	private function build_count_query( $fields, $join, $where, $groupby ) {
+		if ( ! isset( $this->sql ) || '' === $this->sql ) {
+			wc_doing_it_wrong( __FUNCTION__, 'Count query can only be build after main query is built.', '7.3.0' );
+		}
+		$orders_table    = $this->tables['orders'];
+		$this->count_sql = "SELECT COUNT(DISTINCT $fields) FROM  $orders_table $join WHERE $where $groupby";
 	}
 
 	/**
@@ -1014,7 +1034,7 @@ class OrdersTableQuery {
 		}
 
 		if ( $this->limits ) {
-			$this->found_orders  = absint( $wpdb->get_var( 'SELECT FOUND_ROWS()' ) );
+			$this->found_orders  = absint( $wpdb->get_var( $this->count_sql ) ); // phpcs:ignore WordPress.DB.PreparedSQL.NotPrepared
 			$this->max_num_pages = (int) ceil( $this->found_orders / $this->args['limit'] );
 		} else {
 			$this->found_orders = count( $this->orders );

plugins/woocommerce/tests/php/src/Internal/DataStores/Orders/OrdersTableDataStoreTests.php

@@ -1003,6 +1003,48 @@ class OrdersTableDataStoreTests extends WC_Unit_Test_Case {
 		$this->assertEquals( array_slice( $test_orders, 5, 5 ), $query->orders, 'The expected dataset is supplied when paginating through orders.' );
 	}
 
+	/**
+	 * @testdox Test that the query counts works as expected.
+	 *
+	 * @return void
+	 */
+	public function test_cot_query_count() {
+		$this->assertEquals( 0, ( new OrdersTableQuery() )->found_orders, 'We initially have zero orders within our custom order tables.' );
+
+		for ( $i = 0; $i < 30; $i ++ ) {
+			$order = new WC_Order();
+			$this->switch_data_store( $order, $this->sut );
+			if ( 0 === $i % 2 ) {
+				$order->set_billing_address_2( 'Test' );
+			}
+			$order->save();
+		}
+
+		$query = new OrdersTableQuery( array( 'limit' => 5 ) );
+		$this->assertEquals( 30, $query->found_orders, 'Specifying limits still calculate all found orders.' );
+
+		// Count does not change based on the fields that we are fetching.
+		$query = new OrdersTableQuery(
+			array(
+				'fields' => 'ids',
+				'limit'  => 5,
+			)
+		);
+		$this->assertEquals( 30, $query->found_orders, 'Fetching specific field does not change query count.' );
+
+		$query = new OrdersTableQuery(
+			array(
+				'field_query' => array(
+					array(
+						'field' => 'billing_address_2',
+						'value' => 'Test',
+					),
+				),
+			)
+		);
+		$this->assertEquals( 15, $query->found_orders, 'Counting orders with a field query works.' );
+	}
+
 	/**
 	 * @testDox Test the `get_order_count()` method.
 	 */