get_index_columns using SHOW INDEX FROM query instead of information_schema (#36427)

* get_index_columns using SHOW INDEX FROM query instead of information_schema

* returning empty array for clarity

* ignore interpolated query

* adding changelog entry

* Adding unit tests for the updated DatabaseUtil::get_index_columns()

---------

Co-authored-by: Michael Pretty <prettyboymp@Michaels-MacBook-Pro.local>

plugins/woocommerce/changelog/fix-36374-use-show-index-instead-of-db-schema

@@ -0,0 +1,4 @@
+Significance: minor
+Type: fix
+
+Convert DatabaseUtil::get_index_columns() to use SHOW INDEX FROM instead of INFORMATION_SCHEMA query

plugins/woocommerce/src/Internal/Utilities/DatabaseUtil.php

@@ -126,15 +126,14 @@ class DatabaseUtil {
 			$index_name = 'PRIMARY';
 		}
 
-		// phpcs:disable WordPress.DB.PreparedSQL
-		return $wpdb->get_col(
-			"
-SELECT column_name FROM INFORMATION_SCHEMA.STATISTICS
-WHERE table_name='$table_name'
-AND table_schema='" . DB_NAME . "'
-AND index_name='$index_name'"
-		);
-		// phpcs:enable WordPress.DB.PreparedSQL
+		// phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared
+		$results = $wpdb->get_results( $wpdb->prepare( "SHOW INDEX FROM $table_name WHERE Key_name = %s", $index_name ) );
+
+		if ( empty( $results ) ) {
+			return array();
+		}
+
+		return array_column( $results, 'Column_name' );
 	}
 
 	/**

plugins/woocommerce/tests/php/src/Internal/Utilities/DatabaseUtilTest.php

@@ -0,0 +1,60 @@
+<?php
+/**
+ * Tests for the DatabaseUtil utility.
+ */
+
+use Automattic\WooCommerce\Internal\Utilities\DatabaseUtil;
+
+/**
+ * Tests relating to DatabaseUtil.
+ */
+class DatabaseUtilTest extends WC_Unit_Test_Case {
+
+	/**
+	 * @var DatabaseUtil
+	 */
+	private $sut;
+
+	/**
+	 * Set-up subject under test.
+	 */
+	public function set_up() {
+		$this->sut = wc_get_container()->get( DatabaseUtil::class );
+		parent::set_up();
+	}
+
+	/**
+	 * @testdox Test that get_index_columns() will default to return the primary key index columns.
+	 */
+	public function test_get_index_columns_returns_primary_index_by_default() {
+		global $wpdb;
+
+		$this->assertEquals(
+			array( 'product_id' ),
+			$this->sut->get_index_columns( $wpdb->prefix . 'wc_product_meta_lookup' )
+		);
+	}
+
+	/**
+	 * @testdox Test that get_index_columns() will return an array containing all columns of a multi-column index.
+	 */
+	public function test_get_index_columns_returns_multicolumn_index() {
+		global $wpdb;
+
+		$this->assertEquals(
+			array( 'min_price', 'max_price' ),
+			$this->sut->get_index_columns( $wpdb->prefix . 'wc_product_meta_lookup', 'min_max_price' )
+		);
+	}
+
+	/**
+	 * @testdox Test that giving a non-existent index name to get_index_columns() will return an empty array.
+	 */
+	public function test_get_index_columns_returns_empty_for_invalid_index() {
+		global $wpdb;
+
+		$this->assertEmpty(
+			$this->sut->get_index_columns( $wpdb->prefix . 'wc_product_meta_lookup', 'invalid_index_name' )
+		);
+	}
+}