[release-19.0] Implement temporal comparisons (#17826) (#17852)

[mhamza15]

Description

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[Copilot]

PR Overview

This pull request implements temporal comparison support by updating date/time conversion functions and multi‐comparison logic, while extending support for the TIMESTAMP type throughout the evalengine. Key changes include:

• Updates to conversion functions (e.g. Convert_xDTs, toTimestamp) to treat TIMESTAMP and DATETIME uniformly where appropriate.
• Refactoring of multi‐comparison operators in fn_compare.go so that temporal values are handled correctly via both direct temporal comparisons and string-fallback conversions.
• Adjustments in bind variable, column push, and test functions to incorporate the new TIMESTAMP support.

Reviewed Changes

File	Description
go/vt/vtgate/evalengine/compiler_asm.go	Modified Collate signature and added new conversion and multi‐comparison functions for TIMESTAMP.
go/vt/vtgate/evalengine/eval_temporal.go	Updated temporal conversion logic to combine DATETIME and TIMESTAMP handling.
go/vt/vtgate/evalengine/expr_bvar.go and expr_column.go	Split handling of DATETIME and TIMESTAMP in bind variable and column push operations.
go/vt/vtgate/evalengine/compiler.go	Extended compileToTemporal and compileToDecimal to support TIMESTAMP conversions.
go/vt/vtgate/evalengine/compiler_asm_push.go	Added push_timestamp routines for TIMESTAMP column and bind variable support.
go/vt/vtgate/evalengine/fn_compare.go	Major refactoring of multi-comparison functions to support temporal types including TIMESTAMP.
go/vt/vtgate/evalengine/fn_time.go	Adjusted various time functions to include sqltypes.Timestamp in their conversion case lists.

Copilot reviewed 15 out of 15 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (5)

go/vt/vtgate/evalengine/compiler_asm.go:770

• Ensure that all Collate calls and related usage are updated to accept TypedCollation consistently, and verify that this change does not inadvertently alter collation behavior.

func (asm *assembler) Collate(col collations.TypedCollation) {

go/vt/vtgate/evalengine/compiler_asm.go:1173

• Verify that the logic in Convert_xDTs properly distinguishes TIMESTAMP from DATETIME conversions and that downstream usage of the converted values correctly reflects TIMESTAMP semantics.

func (asm *assembler) Convert_xDTs(offset, prec int, allowZero bool) {

go/vt/vtgate/evalengine/eval_temporal.go:108

• Confirm that toTimestamp handles all expected SQL types (including DATETIME, DATE, and TIMESTAMP) consistently, to ensure there are no inconsistencies in temporal comparisons and conversions.

func (e *evalTemporal) toTimestamp(l int, now time.Time) *evalTemporal {

go/vt/vtgate/evalengine/fn_compare.go:260

• Review the fallback mechanism in compareAllTemporalAsString to ensure it correctly converts temporal values to VARCHAR when direct temporal comparison is not feasible.

func compareAllTemporalAsString(f func(env *ExpressionEnv, arg eval, prec int) *evalTemporal) multiComparisonFunc {

go/vt/vtgate/evalengine/fn_time.go:311

• Ensure that including sqltypes.Timestamp in time conversion switch cases is semantically correct and that the conversion functions handle timestamp-specific edge cases appropriately.

case sqltypes.Datetime, sqltypes.Date, sqltypes.Time, sqltypes.Timestamp:

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