feat: optimise outer joins

[systay]

Description

This PR introduces enhancements to the query planner, enabling the push down of LIMIT clauses to the right-hand side (RHS) of joins, including outer joins, when the expressions permit. Additionally, logic has been added to push down LIMIT under routes when applicable.

These changes aim to optimize query efficiency and resource utilization, ensuring faster query processing and lower overhead in the presence of outer joins.

For inner joins, we can't push down LIMIT on the left-hand side (LHS) of our nested loop joins (ApplyJoin). For a join to produce a row, we need a match on both sides, and we don't know how many rows we need from the LHS for the join to produce sufficient rows to satisfy the user-set LIMIT. The best we can do is push a LIMIT on the RHS and then also LIMIT on top of the join.

On the other hand, for left outer joins, we know that every row from the LHS will be returned from the join. This means we can push down the LIMIT to the LHS of the join. This is really good and important - we want to run the RHS of the join as few times as possible.

Example

SELECT * 
FROM user 
  JOIN user_extra ON user.col = user_extra.col
LIMIT 10

The first, unoptimized version of the query with the LIMIT looks like this:

Limit
└── ApplyJoin
    ├── Route
    │   └── Table (user.user)
    └── Route
        └── Filter (joinCondition)
            └── Table (user.user_extra)

Now we rewrite by doing iterative tree rewrites until we reach a fixed point. It will look something like this:

Limit (1)
└── ApplyJoin
    ├── Limit (2)
    │   └── Route
    │       └── Limit (3)
    │           └── Table (user.user)
    └── Limit (4)
        └── Route
            └── Limit (5)
                └── Filter (joinCondition)
                    └── Table (user.user_extra)

Here are the reasons for the different LIMIT operations:

1. This is the top LIMIT. It ensures that the final results do not contain more than 10 rows.
2. We want to run as few rows through our ApplyJoin as possible, so if the Route under is not a single-sharded Route, we add a LIMIT on top of the LHS of the JOIN.
3. We know we don't need more than 10 rows from each shard, so we can push down the LIMIT under the route. 🎉
4. Similar to (2) - if the route under is touching multiple shards, we keep a LIMIT on top of the Route to minimize the number of rows processed by the join.
5. Finally, we know we don't need more than 10 rows from any shard on the RHS of the join.

Related Issue(s)

Fixes #15828

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required**

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 86.84211% with 20 lines in your changes are missing coverage. Please review.

Project coverage is 68.25%. Comparing base (a1edaee) to head (4a6ccd1).
Report is 3 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/engine/simple_projection.go	57.89%	8 Missing ⚠️
go/vt/vtgate/planbuilder/operators/limit.go	14.28%	6 Missing ⚠️
.../vt/vtgate/planbuilder/operators/query_planning.go	92.85%	5 Missing ⚠️
go/vt/vtgate/planbuilder/operators/rewriters.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15840      +/-   ##
==========================================
+ Coverage   68.24%   68.25%   +0.01%     
==========================================
  Files        1562     1562              
  Lines      197171   197300     +129     
==========================================
+ Hits       134550   134669     +119     
- Misses      62621    62631      +10     

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[arthurschreiber]

Would it make sense to define this on the JoinOp type? I'm not really sure when we prefer that over a standalone function, so I'm asking mostly out of curiosity. 🙇‍♂️

[systay]

I did consider that, but that forces all implementations to have to have an implementation of this method, and that seems silly.

If golang had some way of defining method implementations on interfaces, I'd use that

[frouioui]

There seems to be an issue with TestAliasesInOuterJoinQueries when running an outer join where the column names returned are different than what MySQL is sending back. Instead of "t0", "t1", "col" we are getting "t0", "t0", "col"

[harshit-gangal]

There is no end to end test for this change.
I see MySQL syntax error for

select * from t limit 1 + 2;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '+ 2' at line 1

[harshit-gangal]

I think we can still do better here by changing a little more on simple projection like checking len(op.Source.GetColumns(ctx)) >= len(cols). And Simple project truncates from the end.

can leave a TODO here if you think it make sense.

[systay]

the columns need to be of the expected size. we should not return more columns than what the user asked for

[harshit-gangal]

yes, we can do again minimal work of truncate. not important for this PR