ebnf user variable

[dveeden]

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Closes #17046

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[dveeden]

Using ":=" and multiple variables all works in TiDB:

sql> SET GLOBAL max_connections := 500, tidb_use_plan_baselines := ON;
Query OK, 0 rows affected (0.0004 sec)

There is an alternative SET @@global.max_connections = 500. Not sure if we should add that.

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Timeline:

• 2024-04-12 03:52:37.459327463 +0000 UTC m=+1193618.986868002: ☑️ agreed by hfxsd.

[hfxsd]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.0: #17192.