From 7ba59300aed53c800b7d532a3f5505a928f2020a Mon Sep 17 00:00:00 2001
From: Sukhendu Sekhar Guria <sukhenduguria2017@gmail.com>
Date: Wed, 15 Jul 2026 12:59:30 +0530
Subject: [PATCH] Taxonomy: Improve AND term query performance

---
 src/wp-includes/class-wp-tax-query.php | 23 +++++++
 tests/phpunit/tests/term/taxQuery.php  | 94 ++++++++++++++++++++++++++
 2 files changed, 117 insertions(+)

diff --git a/src/wp-includes/class-wp-tax-query.php b/src/wp-includes/class-wp-tax-query.php
index c6ec3258cc201..db2f944206737 100644
--- a/src/wp-includes/class-wp-tax-query.php
+++ b/src/wp-includes/class-wp-tax-query.php
@@ -451,6 +451,29 @@ public function get_sql_for_clause( &$clause, $parent_query ) {
 				return $sql;
 			}
 
+			/*
+			 * Individual joins avoid a correlated subquery for small term intersections.
+			 * Limit total taxonomy joins to three because additional joins can cost more
+			 * than the subquery for common terms.
+			 */
+			if ( 3 >= count( $terms ) + count( $this->table_aliases ) ) {
+				foreach ( $terms as $term ) {
+					$i     = count( $this->table_aliases );
+					$alias = $i ? 'tt' . $i : $wpdb->term_relationships;
+
+					$this->table_aliases[] = $alias;
+
+					$join  = " LEFT JOIN $wpdb->term_relationships";
+					$join .= $i ? " AS $alias" : '';
+					$join .= " ON ($this->primary_table.$this->primary_id_column = $alias.object_id)";
+
+					$sql['join'][]  = $join;
+					$sql['where'][] = "$alias.term_taxonomy_id IN ($term)";
+				}
+
+				return $sql;
+			}
+
 			$num_terms = count( $terms );
 
 			$terms = implode( ',', $terms );
diff --git a/tests/phpunit/tests/term/taxQuery.php b/tests/phpunit/tests/term/taxQuery.php
index 40f1d29612e70..aab8214a9bcdf 100644
--- a/tests/phpunit/tests/term/taxQuery.php
+++ b/tests/phpunit/tests/term/taxQuery.php
@@ -505,6 +505,100 @@ public function test_get_sql_operator_and_empty_terms() {
 		_unregister_taxonomy( 'wptests_tax' );
 	}
 
+	/**
+	 * @ticket 16706
+	 */
+	public function test_get_sql_operator_and_uses_joins_for_three_or_fewer_terms() {
+		$tq = new WP_Tax_Query(
+			array(
+				array(
+					'field'    => 'term_taxonomy_id',
+					'operator' => 'AND',
+					'terms'    => range( 1, 3 ),
+				),
+			)
+		);
+
+		global $wpdb;
+		$sql = $tq->get_sql( $wpdb->posts, 'ID' );
+
+		$this->assertSame( 3, substr_count( $sql['join'], 'JOIN' ) );
+		$this->assertStringNotContainsString( 'SELECT COUNT(1)', $sql['where'] );
+	}
+
+	/**
+	 * @ticket 16706
+	 */
+	public function test_get_sql_operator_and_uses_subquery_for_more_than_three_terms() {
+		$tq = new WP_Tax_Query(
+			array(
+				array(
+					'field'    => 'term_taxonomy_id',
+					'operator' => 'AND',
+					'terms'    => range( 1, 4 ),
+				),
+			)
+		);
+
+		global $wpdb;
+		$sql = $tq->get_sql( $wpdb->posts, 'ID' );
+
+		$this->assertSame( '', $sql['join'] );
+		$this->assertStringContainsString( 'SELECT COUNT(1)', $sql['where'] );
+	}
+
+	/**
+	 * @ticket 16706
+	 */
+	public function test_get_sql_operator_and_limits_total_joins_across_clauses() {
+		$tq = new WP_Tax_Query(
+			array(
+				array(
+					'field'    => 'term_taxonomy_id',
+					'operator' => 'AND',
+					'terms'    => range( 1, 2 ),
+				),
+				array(
+					'field'    => 'term_taxonomy_id',
+					'operator' => 'AND',
+					'terms'    => range( 3, 4 ),
+				),
+			)
+		);
+
+		global $wpdb;
+		$sql = $tq->get_sql( $wpdb->posts, 'ID' );
+
+		$this->assertSame( 2, substr_count( $sql['join'], 'JOIN' ) );
+		$this->assertStringContainsString( 'SELECT COUNT(1)', $sql['where'] );
+	}
+
+	/**
+	 * @ticket 16706
+	 */
+	public function test_get_sql_operator_and_counts_existing_in_join_toward_limit() {
+		$tq = new WP_Tax_Query(
+			array(
+				array(
+					'field'    => 'term_taxonomy_id',
+					'operator' => 'IN',
+					'terms'    => 1,
+				),
+				array(
+					'field'    => 'term_taxonomy_id',
+					'operator' => 'AND',
+					'terms'    => range( 2, 4 ),
+				),
+			)
+		);
+
+		global $wpdb;
+		$sql = $tq->get_sql( $wpdb->posts, 'ID' );
+
+		$this->assertSame( 1, substr_count( $sql['join'], 'JOIN' ) );
+		$this->assertStringContainsString( 'SELECT COUNT(1)', $sql['where'] );
+	}
+
 	/**
 	 * @ticket 18105
 	 * @covers WP_Tax_Query::get_sql