Do not load table stats when booting vttablet.

[arthurschreiber]

Description

As part of the bootup process, vttablet reloads the schema engine, with table statistics information like table size, via the BaseShowTablesWithSizes query. This query is relatively expensive, but calling it once on vttablet bootup doesn't really seem much like an issue in production environments.

But in vtcombo, where a lot of vttablet instances are booted up on startup, all these BaseShowTablesWithSizes calls can add up, especially when there's a lot of tables / tablespaces.

In our CI system, where we use vtcombo, just the process of starting vtcombo on MySQL 8.0 takes 70 (!) seconds. On MySQL 5.7, it takes 26 seconds.

I already prepared a PR to halve the cost of running BaseShowTablesWithSizes on MySQL 8.0. But you know what's even better than halving the cost of this query? Not even running it in the first place!

This pull request proposes to skip the table size collection when booting up vttablet. This will improve the startup time, with virtually no negative side effect. The table statistics will be collected during the next regular schema engine reload interval.

This brings vtcombo bootup time in our CI system down to roughly 15 seconds, both on MySQL 5.7 and MySQL 8.0.

I'd really love to see this backported, because it's a massive performance improvement for everyone using vtcombo without any noticeable negative side effects, and it'd reduce the amount of patches I need to juggle until we get to a more recent version of Vitess. 😬

Related Issue(s)

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[harshit-gangal]

Unit Tests are failing, those need to be fixed as the test expectation about the query have changed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.45%. Comparing base (f118ba2) to head (8e4b089).
Report is 58 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15715      +/-   ##
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+ Coverage   68.40%   68.45%   +0.05%     
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  Files        1556     1559       +3     
  Lines      195121   196515    +1394     
==========================================
+ Hits       133479   134532    +1053     
- Misses      61642    61983     +341     

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