The TABLE_SCHEMA and TABLE_NAME are already guaranteed to have the same value in COLUMNS and UNIQUES because of the WHEREs in each query. Dropping it from the ON clause makes it complete much faster.
On (at least) MySQL 5.6 installs with thousands of tables, this query completes in a few seconds without the additional join conditions, but takes more than a minute with it.
There is no need to call `applyColumnTypes` more than once for the same
`databaseName` and `tableName`, it is just move the additional
`columnList` to the first call.
The given `columnLists` may not contain all columns available in the
given table. This patch prevents the code to fail. Before this patch the
code was panicking whenever it was trying to set a value into a `nil`
object, e.g. `columnList.GetColumn('non-existant').Type = SomeType`.
For some reason the application was not completely failing but as a
side-effect any column after the non-existant column would never get its
Type properly set.
Both Percona and Maria allow MySQL to be configured to listen on an extra port when their thread pool is enable.
* https://www.percona.com/doc/percona-server/5.7/performance/threadpool.html
* https://mariadb.com/kb/en/the-mariadb-library/thread-pool-in-mariadb-51-53/
This is valuable because if the table has a lot of traffic (read or write load), gh-ost can end up starving the thread pool as incomming connections are immediately blocked.
By using gh-ost on the extra port, MySQL locking will still behave the same, but MySQL will keep a dedicated thread for each gh-ost connection.
When doing this, it's important to inspect the extra-max-connections variable. Both Percona and Maria default to 1, so gh-ost may easily exceed with its threads.
An example local run using this
```
$ mysql -S /tmp/mysql_sandbox20393.sock -e "select @@global.port, @@global.extra_port"
+---------------+---------------------+
| @@global.port | @@global.extra_port |
+---------------+---------------------+
| 20393 | 30393 |
+---------------+---------------------+
./bin/gh-ost \
--initially-drop-ghost-table \
--initially-drop-old-table \
--assume-rbr \
--port="20395" \
--assume-master-host="127.0.0.1:30393" \
--max-load=Threads_running=25 \
--critical-load=Threads_running=1000 \
--chunk-size=1000 \
--max-lag-millis=1500 \
--user="gh-ost" \
--password="gh-ost" \
--database="test" \
--table="mytable" \
--verbose \
--alter="ADD mynewcol decimal(11,2) DEFAULT 0.0 NOT NULL" \
--exact-rowcount \
--concurrent-rowcount \
--default-retries=120 \
--panic-flag-file=/tmp/ghost.panic.flag \
--postpone-cut-over-flag-file=/tmp/ghost.postpone.flag \
--execute
```
