Update MySQL.php

[josemariagomezroncero]

Improves query speed for MySQL 5.7, MariaDB 10.3+

Questions	Answers
Description?	Please be specific when describing the PR. Every detail helps: versions, browser/server configuration, specific module/theme, etc. Feel free to add more information below this table.
Type?	improvement
Deprecations?	no
Fixed ticket?	Fixes PrestaShop/Prestashop#{issue number here}.
How to test?	A database with tens of thousands of products is required. MariaDB 11+ is required.
When using a faceted filter and selecting more than one feature, the query execution time exceeds one minute. With this improvement, that time is reduced to hundredths of a second.

[ps-jarvis]

Hello @josemariagomezroncero!

This is your first pull request on ps_facetedsearch repository of the PrestaShop project.

Thank you, and welcome to this Open Source community!