Chunk query that can potentially OOM

Unbound query has potential to return millions of
rows, and it's fairly easy to run into memory problems.

e.g. we had a problem with a ~4GB DB that was returning
13 million rows to check for the `meta_value` column.

The fix loops through the table fetching 1000 at a time.
Added ORDER BY statement, so we can safely iterate through
the primary keys.

Using ASC ordering, so the new items come last,
in case something gets added while it's running.

Author: aidvu

src/Search_Replace_Command.php

@@ -533,46 +533,55 @@ private function php_handle_col( $col, $primary_keys, $table, $old, $new ) {
 		$col_sql          = self::esc_sql_ident( $col );
 		$where            = $this->regex ? '' : " WHERE $col_sql" . $wpdb->prepare( ' LIKE BINARY %s', '%' . self::esc_like( $old ) . '%' );
 		$primary_keys_sql = implode( ',', self::esc_sql_ident( $primary_keys ) );
+		$order_by_keys = array_map(
+			function( $key ) {
+				return "{$key} ASC";
+			},
+			$primary_keys
+		);
+		$order_by_sql = "ORDER BY " . implode( ',', self::esc_sql_ident( $order_by_keys ) );
+		$limit = 1000;
+		$offset = 0;
 
 		// phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared -- escaped through self::esc_sql_ident
-		$rows = $wpdb->get_results( "SELECT {$primary_keys_sql} FROM {$table_sql} {$where}" );
-
-		foreach ( $rows as $keys ) {
-			$where_sql = '';
-			foreach ( (array) $keys as $k => $v ) {
-				if ( strlen( $where_sql ) ) {
-					$where_sql .= ' AND ';
+		while ( $rows = $wpdb->get_results( "SELECT {$primary_keys_sql} FROM {$table_sql} {$where} {$order_by_sql} LIMIT {$limit} OFFSET {$offset}" ) ) {
+			foreach ( $rows as $keys ) {
+				$where_sql = '';
+				foreach ( (array) $keys as $k => $v ) {
+					if ( strlen( $where_sql ) ) {
+						$where_sql .= ' AND ';
+					}
+					$where_sql .= self::esc_sql_ident( $k ) . ' = ' . self::esc_sql_value( $v );
 				}
-				$where_sql .= self::esc_sql_ident( $k ) . ' = ' . self::esc_sql_value( $v );
-			}
 
-			// phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared -- escaped through self::esc_sql_ident
-			$col_value = $wpdb->get_var( "SELECT {$col_sql} FROM {$table_sql} WHERE {$where_sql}" );
-
-			if ( '' === $col_value ) {
-				continue;
-			}
+				// phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared -- escaped through self::esc_sql_ident
+				$col_value = $wpdb->get_var( "SELECT {$col_sql} FROM {$table_sql} WHERE {$where_sql}" );
 
-			$value = $replacer->run( $col_value );
+				if ( '' === $col_value ) {
+					continue;
+				}
 
-			if ( $value === $col_value ) {
-				continue;
-			}
+				$value = $replacer->run( $col_value );
 
-			if ( $this->log_handle ) {
-				$this->log_php_diff( $col, $keys, $table, $old, $new, $replacer->get_log_data() );
-				$replacer->clear_log_data();
-			}
+				if ( $value === $col_value ) {
+					continue;
+				}
 
-			if ( $this->dry_run ) {
-				$count++;
-			} else {
-				$where = array();
-				foreach ( (array) $keys as $k => $v ) {
-					$where[ $k ] = $v;
+				if ( $this->log_handle ) {
+					$this->log_php_diff( $col, $keys, $table, $old, $new, $replacer->get_log_data() );
+					$replacer->clear_log_data();
 				}
 
-				$count += $wpdb->update( $table, array( $col => $value ), $where );
+				if ( $this->dry_run ) {
+					$count++;
+				} else {
+					$where = array();
+					foreach ( (array) $keys as $k => $v ) {
+						$where[ $k ] = $v;
+					}
+
+					$count += $wpdb->update( $table, array( $col => $value ), $where );
+				}
 			}
 		}