Add default collation to VTGate and VTTablet

[frouioui]

Description

This pull request does two things:

• Changing the default charset from utf8 to utf8mb4
• Adding a default collation to both VTGate and VTTablet

We are changing the default charset from utf8 (utf8mb3) to utf8mb4 because utf8mb4 is way more optimized than its predecessor utf8mb3. Another reason, utf8mb4 supports more characters. With utf8mb4 being a superset of utf8mb3, queries that were previously relying on utf8mb3 should not have a problem with this new charset.

Default Collation

We are adding a default collation to both VTGate and VTTablet, this is done to let us do string comparisons at the VTGate level. It also ensures that we are using the same connection collation as the one between VTTablet and MySQL. For both components, the collation can be set using the -collation (for VTGate) and -db_collation (for VTTablet). If the flag is not specified, it will default to the default collation of utf8mb4 in regards to the underlying database version we are running (MySQL57, MySQL80, etc).

The default collation for certain charset varies depending on the MySQL version we are running, for this reason, the VTTablet is responsible for telling VTGate which MySQL version is being used.

VTGate

At start time, VTGate subscribes to a health check and waits until a VTTablet indicates which MySQL version is used. Once the MySQL version is known in VTGate, VTGate will create its own collation environment that contains the default collation to use and the collations.Environment that will allow us to lookup other collations when planning/executing queries.

Before each execution, the default collation is added to the ExecuteOptions so the VTTablet can make sure collations are all the same from the client to MySQL.

VTTablet

When opening a new connection with a MySQL server, VTTablet is responsible for setting the connection collation properly using the collation/charset the user has defined, and again, in regards to the underlying MySQL version we are connecting to.

Documentation

• Default collation documentation website#879

Checklist

• Tests were added or are not required
• Documentation was added or is not required

[systay]

❤️ these comments

[vmg]

This is looking great Floflo. I don't like the collation environment in the Conn Params though. It doesn't feel right, again, because it doesn't serialize as JSON, which the conn params must do. Let's come up with a different place to store it. Hit me up in Slack if you need help.

[frouioui]

This is looking great Floflo. I don't like the collation environment in the Conn Params though. It doesn't feel right, again, because it doesn't serialize as JSON, which the conn params must do. Let's come up with a different place to store it. Hit me up in Slack if you need help.

Your two comments are addressed via a56fe1d

[vmg]

Paired on this with @frouioui this morning. Lookin' gucci. 🇫🇷👑