Performance: use IsSingleShard() check in pushDerived instead of just engine.EqualUnique opcode

[ChaitanyaD48]

Description

This PR enhances the pushDerived function in route_planning.go to consider all single-shard routing scenarios, not just those using the engine.EqualUnique opcode.

IsSingleShard() method checks if a route is a single shard route by looking at its opcode. The method returns true for the following opcodes: Unsharded, DBA, Next, EqualUnique, and Reference.

Reference - vitess/go/vt/vtgate/engine/routing.go

func (code Opcode) IsSingleShard() bool {
	switch code {
	case Unsharded, DBA, Next, EqualUnique, Reference:
		return true
	}
	return false
}

Related Issue(s)

Fixes #18952

Checklist

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[arthurschreiber]

Can you also add test cases that show the impact of this change?

[ChaitanyaD48]

Can you also add test cases that show the impact of this change?

Hi @arthurschreiber
I’ve been testing a few scenarios to validate the impact of this change, and I think this testcase demonstrates the expected behavior -

[
  {
    "comment": "Derived table pushdown optimization",
    "query": "SELECT * FROM (SELECT DISTINCT id, col FROM user.ref WHERE id = 1 LIMIT 10) as sub",
    "plan": {
      "Type": "Passthrough",
      "QueryType": "SELECT",
      "Original": "SELECT * FROM (SELECT DISTINCT id, col FROM user.ref WHERE id = 1 LIMIT 10) as sub",
      "Instructions": {
        "OperatorType": "Route",
        "Variant": "Reference",
        "Keyspace": {
          "Name": "user",
          "Sharded": true
        },
        "FieldQuery": "select id, col from (select id, col from ref where 1 != 1) as sub where 1 != 1",
        "Query": "select id, col from (select distinct id, col from ref where id = 1 limit 10) as sub"
      },
      "TablesUsed": [
        "user.ref"
      ]
    }
  }
]

Properties of the query -

• user.ref is a Reference table (replicated across shards)
• Subquery contains LIMIT, making the operator non-mergeable thus !op.IsMergeable(ctx) condition becomes true.
• Routing opcode is Reference, which is now recognized as single-shard thus !innerRoute.Routing.OpCode().IsSingleShard() condition becomes false

These are the test results in both scenarios

Previous Case (EqualUnique-only):

• isMergeable = false
• opcode = Reference
• Pushdown is blocked, even though the route is actually single-shard
• vtgate must perform DISTINCT + LIMIT

Output -

Refactored Case (using IsSharded):

• IsSingleShard() = true for Reference variant
• Pushdown is allowed
• MySQL executes the entire subquery

Output -

What do you think?

[ChaitanyaD48]

Hi @arthurschreiber just following up on my previous comment.
Let me know what do you think of the test case provided.
Thanks!

[arthurschreiber]

@ChaitanyaD48 The test case makes sense to me. 👍

[ChaitanyaD48]

Thanks! @arthurschreiber
I've added the test case to reference_cases.json