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

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 col FROM user.ref WHERE col = 1 LIMIT 10) as sub",
    "plan": {
      "Type": "Passthrough",
      "QueryType": "SELECT",
      "Original": "SELECT * FROM (SELECT DISTINCT col FROM user.ref WHERE col = 1 LIMIT 10) as sub",
      "Instructions": {
        "OperatorType": "Route",
        "Variant": "Reference",
        "Keyspace": {
          "Name": "user",
          "Sharded": true
        },
        "FieldQuery": "select col from (select col from ref where 1 != 1) as sub where 1 != 1",
        "Query": "select col from (select distinct col from ref where col = 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?

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

@ChaitanyaD48 The test case makes sense to me. 👍

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

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.91%. Comparing base (34f9ba0) to head (cca1058).
⚠️ Report is 6 commits behind head on main.

@@           Coverage Diff           @@
##             main   #18974   +/-   ##
=======================================
  Coverage   69.91%   69.91%           
=======================================
  Files        1611     1611           
  Lines      216178   216178           
=======================================
+ Hits       151134   151142    +8     
+ Misses      65044    65036    -8     

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