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planner, sessionctx: enable indexMerge by default #29617

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planner, sessionctx: enable indexMerge by default #29617

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Yui-Song
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@Yui-Song Yui-Song commented Nov 9, 2021

enable tidb_enable_index_merge by default

What problem does this PR solve?

Issue Number: close #29597

Problem Summary:

What is changed and how it works?

Enable tidb_enable_index_merge by default

Check List

Tests

  • Unit test
  • Integration test
  • Manual test (add detailed scripts or steps below)
  • No code

Side effects

  • Performance regression: Consumes more CPU
  • Performance regression: Consumes more Memory
  • Breaking backward compatibility

Documentation

  • Affects user behaviors
  • Contains syntax changes
  • Contains variable changes
  • Contains experimental features
  • Changes MySQL compatibility

Release note

Change tidb_enable_index_merge to "on"

@Yui-Song
enable tidb_enable_index_merge by default
34c07a8 
enable tidb_enable_index_merge by default
@Yui-Song Yui-Song requested a review from a team as a code owner November 9, 2021 11:09
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@ti-chi-bot ti-chi-bot added the release-note Denotes a PR that will be considered when it comes time to generate release notes. label Nov 9, 2021
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@ti-chi-bot ti-chi-bot added the size/M Denotes a PR that changes 30-99 lines, ignoring generated files. label Nov 9, 2021
@Yui-Song Yui-Song requested a review from winoros November 9, 2021 11:10
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sre-bot commented Nov 9, 2021

Please follow PR Title Format:

  • pkg [, pkg2, pkg3]: what is changed

Or if the count of mainly changed packages are more than 3, use

  • *: what is changed

After you have format title, you can leave a comment /run-check_title to recheck it

@Yui-Song Yui-Song requested a review from qw4990 November 9, 2021 11:10
@Yui-Song Yui-Song changed the title planner, sessionctx: enable tidb_enable_index_merge by default planner, sessionctx: enable indexMerge by default Nov 9, 2021
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Yui-Song commented Nov 9, 2021

/run-check_title

morgo
morgo reviewed Nov 9, 2021
View reviewed changes
sessionctx/variable/sysvar.go
@@ -1075,7 +1075,7 @@ var defaultSysVars = []*SysVar{
s.SetEnableCascadesPlanner(TiDBOptOn(val))
return nil
}},
{Scope: ScopeGlobal | ScopeSession, Name: TiDBEnableIndexMerge, Value: Off, Type: TypeBool, SetSession: func(s *SessionVars, val string) error {
{Scope: ScopeGlobal | ScopeSession, Name: TiDBEnableIndexMerge, Value: On, Type: TypeBool, SetSession: func(s *SessionVars, val string) error {
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This will only change the default for new installations. I believe this is fine, but I will wait for you to confirm (and update the release note).

If you'd like to change for existing installs, a bootstrap task is needed (session/bootstrap.go).

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(1) Please make sure this works for 4.0 to 5.0 upgrade too, since from analyze-2.0 feature we found it only works for intra-5.0 upgrade but does not work for the 4.0 to 5.0 upgrade which changes the default anyway no matter it is existing or new installs;
(2) Also @guo-shaoge is working on GA index merge in Sprint 6. I'd suggest to make this change together with index merge GA feature.

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I think just change the default for new installations is enough. Change for existing installs will results in plan changes, which will give the user "surprise". That's not what I expected.
And I will talk to @guo-shaoge later.

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The problem here is that approach we identify whether it's an existing or new install only works for 5.x but not 4.x, which means all the existing system from 4.x will be impacted and default will be changed after upgrading to 5.x.

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  1. config, session: promise the compatibility of oom-action when upgrading #22102 changed the default oom-action, which works for all situations(3.0 to 4.0 and 4.0 to 5.0).
    But oom-action is a config item, I am not sure it works for sysvars.
  2. IMO, it's better to enable index merge after it's official GA. We are working on it.

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/run-common-test
/run-integration-copr-test
/run-integration-common-test

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@Yui-Song: PR needs rebase.

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@ti-chi-bot ti-chi-bot added the needs-rebase Indicates a PR cannot be merged because it has merge conflicts with HEAD. label Dec 31, 2021
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Enabled here: #30650 . Guess we can close this pr @Yui-Song

@Yui-Song Yui-Song closed this Apr 20, 2022
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enable tidb_enable_index_merge by default
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