Preserve constants when doing CopyOnRewrite

[andyedison]

Description

This PR is to address a query plan regression we observed when migrating from v20 to v21. It preserves literals within the simplifyPredicates function so that it doesn't break routing optimizations

Related Issue(s)

#18987

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

Deployment Notes

AI Disclosure

This PR was written with the assistance of GitHub Copilot and Claude Opus 4.5 to help identify where the bug could be in the plan parsing, as well as a fix. Executed within environments able to reproduce #18987

[vitess-bot]

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[andyedison]

This code was added in #16689, which landed in v21 - the version in which we saw this query plan regression in #18987

[andyedison]

Still working on test(s) which is why the draft status

[andyedison]

With this fix implemented on our repro fork https://github.com/andyedison/vitess/tree/v23-18987-repro

"Inputs": [
  {
    "OperatorType": "Route",
    "Variant": "None",
    "Keyspace": {
      "Name": "foobar_ks",
      "Sharded": true
    },
    "FieldQuery": "select id, subquery_for_limit.updated_at from (select foobar.id, foobar.updated_at from foobar where 1 != 1) as subquery_for_limit where 1 != 1",
    "OrderBy": "1 DESC",
    "Query": "select id, subquery_for_limit.updated_at from (select foobar.id, foobar.updated_at from foobar where 0) as subquery_for_limit order by subquery_for_limit.updated_at desc limit 100"
  }
]

v23-18987-repro ⇡1 !1 ───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── ✔  5s   23:27:04  
╰─ git --no-pager show                                                     
commit ed3e1ab971e01c09a10c7b55f199d0d9fdb4403f (HEAD -> v23-18987-repro)
Author: Andy Edison <[email protected]>
Date:   Sun Dec 7 23:25:53 2025 -0600

    Proposed fix 1897

diff --git a/go/vt/vtgate/planbuilder/operators/expressions.go b/go/vt/vtgate/planbuilder/operators/expressions.go
index f42ec87404..1309e4cc32 100644
--- a/go/vt/vtgate/planbuilder/operators/expressions.go
+++ b/go/vt/vtgate/planbuilder/operators/expressions.go
@@ -114,7 +114,20 @@ func simplifyPredicates(ctx *plancontext.PlanningContext, in sqlparser.Expr) sql
                        replace = nil
                }
        }
-       output := sqlparser.CopyOnRewrite(in, pre, post, ctx.SemTable.CopySemanticInfo).(sqlparser.Expr)
+       // Custom clone function for CopyOnRewrite that preserves correct constant detection.
+       // When we replace complex expressions with constant literals during simplification,
+       // we intentionally skip copying semantic info (like table dependencies). For example,
+       // when "col = 1 AND 1 = 0" simplifies to "0", the "0" must not inherit dependencies
+       // from "col". Otherwise, IsConstantBool won't recognize it as constant, breaking
+       // routing optimization.
+       cloned := func(from, to sqlparser.SQLNode) {
+               // Don't copy semantic info if the replacement is a constant literal
+               if _, isLiteral := to.(*sqlparser.Literal); isLiteral {
+                       return
+               }
+               ctx.SemTable.CopySemanticInfo(from, to)
+       }
+       output := sqlparser.CopyOnRewrite(in, pre, post, cloned).(sqlparser.Expr)
        if in != output {
                // we need to do this, since one simplification might lead to another
                return simplifyPredicates(ctx, output)

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@3dd1516). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
go/vt/vtgate/planbuilder/operators/expressions.go	80.00%	1 Missing ⚠️

Additional details and impacted files

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[andyedison]

closing in favor of #19003