MySQL loadTables performance

[ronits-cx]

Issue type:

[x] bug report
[ ] feature request
[ ] documentation issue

Database system/driver:

[ ] cordova
[ ] mongodb
[ ] mssql
[x] mysql / mariadb
[ ] oracle
[ ] postgres
[ ] cockroachdb
[ ] sqlite
[ ] sqljs
[ ] react-native
[ ] expo

TypeORM version:

[ ] latest
[ ] @next
[ ] 0.x.x (or put your version here)

Steps to reproduce or a small repository showing the problem:

In `MysqlQueryRunner`, `loadTables` performs queries to `INFORMATION_SCHEMA`, which is not optimized.
MySQL recommends adding a where clause with the schema and table for better performance - https://dev.mysql.com/doc/refman/5.7/en/information-schema-optimization.html.

There are 2 queries with a JOIN that have a where clause on the schema and table, but the where clause is only for one of the tables in the join.
In the foreign key query, this causes an actual bug (which is what https://github.com//pull/6169/files solves).

For large databases with many schemas, this can be a crucial improvement.
In our database, for example, each of these queries takes about **7.5 seconds**.
If a where clause is added to the second table in the join, the query takes around **200 milliseconds**.