fix: Make bulkCreate run in O(N) time. Fixes issue #4247. (#7391)

In generating the bulk insert query, the serials are stored in an
array. This array can potentially be up to size N, which is the number
of rows to be created.

When generating the tuples at a later stage, we walk through the
records one-by-one - O(N) iterations. However, in a single iteration,
we were performing a lookup in the serials array (indexOf costs O(N)),
leading to a O(N^2) runtime and poor performance for large number of
records inserted.

This patch converts the serials array into an object map instead, so
lookups cost O(1) time.

changelog.md

@@ -56,6 +56,7 @@
 - [FIXED] Connection error when fetching OIDs for unspported types in Postgres 8.2 or below [POSTGRES] [#5254](https://github.com/sequelize/sequelize/issues/5254)
 - [FIXED] Expose OptimisticLockError on Sequelize object [#7291](https://github.com/sequelize/sequelize/pull/7291)
 - [FIXED] Deleted paranoid records can be queried in the same second. [#7204](https://github.com/sequelize/sequelize/issues/7204)/[#7332](https://github.com/sequelize/sequelize/pull/7332)
+- [FIXED] `bulkCreate` now runs in O(N) time instead of O(N^2) time. [#4247](https://github.com/sequelize/sequelize/issues/4247)
 
 ## BC breaks:
 - `DATEONLY` now returns string in `YYYY-MM-DD` format rather than `Date` type

lib/dialects/abstract/query-generator.js

@@ -222,7 +222,7 @@ const QueryGenerator = {
 
     const query = 'INSERT<%= ignoreDuplicates %> INTO <%= table %> (<%= attributes %>) VALUES <%= tuples %><%= onDuplicateKeyUpdate %><%= returning %>;';
     const tuples = [];
-    const serials = [];
+    const serials = {};
     const allAttributes = [];
     let onDuplicateKeyUpdate = '';
 
@@ -233,14 +233,14 @@ const QueryGenerator = {
         }
 
         if (rawAttributes[key] && rawAttributes[key].autoIncrement === true) {
-          serials.push(key);
+          serials[key] = true;
         }
       });
     }
 
     for (const attrValueHash of attrValueHashes) {
       tuples.push('(' + allAttributes.map(key => {
-        if (this._dialect.supports.bulkDefault && serials.indexOf(key) !== -1) {
+        if (this._dialect.supports.bulkDefault && serials[key] === true) {
           return attrValueHash[key] || 'DEFAULT';
         }
         return this.escape(attrValueHash[key], rawAttributes[key], { context: 'INSERT' });