On connect, set all variables in a single SET statement

[dolmen]

Description

For faster connection startup this patch use (almost) a single SET statement to set all variables at once instead of a SET statement for each variable.

MySQL doc about the SET statement: https://dev.mysql.com/doc/refman/8.0/en/set-variable.html

Before:

• SET <variable1>=<value1>
• SET NAMES utf8mb4
• SET <variable2>=<value2>
• SET <variable3>=<value3>

Now:

• SET NAMES utf8mb4
• SET <variable1>=<value1>,<variable2>=<value2>,<variable3>=<value3>

This patch should make establishing new connection must faster as it reduce the number of roundtrips necessary before the first query when multiple variables are defined in the DSN.

As a side effect, the SET NAMES statement is always executed first. This will fix the possibility of some SET statements being executed before the right charset is set (for the case where the database default charset is not the one we want to use for the connection, e.g. Latin1).

Here is how to test this patch with your own Go project built with Go modules:

$ go mod edit -replace=github.com/go-sql-driver/mysql=github.com/dolmen/[email protected]

Background: on MySQL 5.7 database hosted on Amazon RDS we noticed that in Performance Insights that some costly queries were hidden by SET statements. Sometimes we saw a SET time_zone=..., sometimes a SET group_concat_max_len=..., sometimes SET sql_mode='TRADITIONAL'.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing: https://travis-ci.org/github/dolmen/mysql/builds/686635632
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[julienschmidt]

LGTM, thanks!

Travis is driving me a little crazy lately... I have no idea why we have two status checks here now and how to make it pass. Restarting the job it is blocked on just updates the other status check.
Could you try to rebase this on the current master before I disable Travis as a required check?

[julienschmidt]

nitpick (not a must): This allocates a new string for every param. Maybe using strings.Builder (and adding the , here) is the better approach?

[dolmen]

I know there are improvement in this area. For this important pull request I wanted to apply the minimum change required. The string concatenation problem was in the original code, so I didn't want to change that and divert the reviewers from the main issue.

Now that this structural change is merged (thanks!), I am now able to propose some further implementation improvements.