"advanced" memory quota variables have no effect

[kolbe]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

set tidb_mem_quota_hashjoin=0;

2. What did you expect to see? (Required)

This variable is "deprecated". A deprecated variable should still have some effect, especially since the documentation still claims that they have some effect: https://docs.pingcap.com/tidb/dev/configure-memory-usage

3. What did you see instead (Required)

These variables should be removed from the documentation if they have no effect.

The source code claims that these variables "do not take any effect anymore", and there's even a TODO to remove them in 4.1. This warning has been in the code for nearly 2 years. Maybe finally time to remove these entirely?

https://github.com/pingcap/tidb/blob/master/sessionctx/variable/session.go#L1904

4. What is your TiDB version? (Required)

Edition: Community
Git Commit Hash: 55f3b24c1c9f506bd652ef1d162283541e428872
Git Branch: heads/refs/tags/v5.4.0
UTC Build Time: 2022-01-25 08:41:03
GoVersion: go1.16.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[morgo]

This is an easy fix if this is true: we have a feature called "remove sysvars" now: #28931

@yudongusa Can you confirm that we can remove these variables from tidb master?

[yudongusa]

This is an easy fix if this is true: we have a feature called "remove sysvars" now: #28931

@yudongusa Can you confirm that we can remove these variables from tidb master?

I'll leave this for @XuHuaiyu to double check as this is cap memory usage of hash join operator.

[coderplay]

@XuHuaiyu Please take a look at this as well: #32287

[coderplay]

@XuHuaiyu so does this one: #32288

[coderplay]

4th issue: #32289

[morgo]

I just rememebered I have a related PR on this: #30848 -- The following system variables are 'known' to not be working correctly: TiDBMemQuotaQuery, TiDBMemQuotaHashJoin, TiDBMemQuotaMergeJoin, TiDBMemQuotaSort, TiDBMemQuotaTopn, TiDBMemQuotaIndexLookupReader.

So it makes sense they should just be removed.