Performance improvements for formatting in SqlString

closes mysqljs#1431

Author: nwoltman

Changes.md

@@ -10,6 +10,7 @@ you spot any mistakes.
 * Fix Query stream to emit close after ending #1349 #1350
 * Fix type cast for BIGINT columns when number is negative #1376
 * Performance improvements for array/object escaping in SqlString #1331
+* Performance improvements for formatting in SqlString #1431
 * Performance improvements for string escaping in SqlString #1390
 * Support Node.js 6.x
 * Update `bignumber.js` to 2.3.0

lib/protocol/SqlString.js

@@ -72,20 +72,40 @@ SqlString.arrayToList = function arrayToList(array, timeZone) {
 };
 
 SqlString.format = function format(sql, values, stringifyObjects, timeZone) {
-  values = values == null ? [] : [].concat(values);
+  if (values == null) {
+    return sql;
+  }
 
-  var index = 0;
-  return sql.replace(/\?\??/g, function(match) {
-    if (index === values.length) {
-      return match;
-    }
+  if (!(values instanceof Array || Array.isArray(values))) {
+    values = [values];
+  }
+
+  var chunkIndex        = 0;
+  var placeholdersRegex = /\?\??/g;
+  var result            = '';
+  var valuesIndex       = 0;
+  var match;
+
+  while (valuesIndex < values.length && (match = placeholdersRegex.exec(sql))) {
+    var value = match[0] === '??'
+        ? SqlString.escapeId(values[valuesIndex])
+        : SqlString.escape(values[valuesIndex], stringifyObjects, timeZone);
 
-    var value = values[index++];
+    result += sql.slice(chunkIndex, match.index) + value;
+    chunkIndex = placeholdersRegex.lastIndex;
+    valuesIndex++;
+  }
+
+  if (chunkIndex === 0) {
+    // Nothing was replaced
+    return sql;
+  }
+
+  if (chunkIndex < sql.length) {
+    return result + sql.slice(chunkIndex);
+  }
 
-    return match === '??'
-      ? SqlString.escapeId(value)
-      : SqlString.escape(value, stringifyObjects, timeZone);
-  });
+  return result;
 };
 
 SqlString.dateToString = function dateToString(date, timeZone) {

test/unit/protocol/test-SqlString.js

@@ -180,5 +180,15 @@ test('SqlString.format', {
 
     var sql = SqlString.format('?', { toString: function () { return 'hello'; } }, true);
     assert.equal(sql, "'hello'");
+  },
+
+  'sql is untouched if no values are provided': function () {
+    var sql = SqlString.format('SELECT ??');
+    assert.equal(sql, 'SELECT ??');
+  },
+
+  'sql is untouched if values are provided but there are no placeholders': function () {
+    var sql = SqlString.format('SELECT COUNT(*) FROM table', ['a', 'b']);
+    assert.equal(sql, 'SELECT COUNT(*) FROM table');
   }
 });