Implement temporal comparisons

[dbussink]

This is currently missing and leads to incorrect types to be returned for LEAST & GREATEST as comparison functions.

There's a little mismatch here in behavior compared to MySQL which I argue is actually a bug in MySQL. In MySQL, a temporal type always has the binary collation:

mysql> select NOW(6), collation(NOW(6));
+----------------------------+-------------------+
| NOW(6)                     | collation(NOW(6)) |
+----------------------------+-------------------+
| 2025-02-19 15:33:21.732301 | binary            |
+----------------------------+-------------------+
1 row in set (0.00 sec)

On MySQL 8.4, this results in:

mysql> select GREATEST(NOW(6), NOW(6)), collation(GREATEST(NOW(6), NOW(6)));
+----------------------------+-------------------------------------+
| GREATEST(NOW(6), NOW(6))   | collation(GREATEST(NOW(6), NOW(6))) |
+----------------------------+-------------------------------------+
| 2025-02-19 15:35:00.921308 | latin1_swedish_ci                   |
+----------------------------+-------------------------------------+
1 row in set (0.00 sec)

But on MySQL 8.0, it returns:

mysql> select GREATEST(NOW(6), NOW(6)), collation(GREATEST(NOW(6), NOW(6)));
+----------------------------+-------------------------------------+
| GREATEST(NOW(6), NOW(6))   | collation(GREATEST(NOW(6), NOW(6))) |
+----------------------------+-------------------------------------+
| 2025-02-19 15:35:00.921308 | utf8mb4_0900_ai_ci                  |
+----------------------------+-------------------------------------+
1 row in set (0.00 sec)

Neither of these collations make sense, because it really should not change the collation and return binary still. That is what Vitess still does with the changes here (hence the addition to the test framework to allow skipping the collation check).

I'll also report the issue upstream to make it behave correctly there as well.

Marked this also for backporting since using these types today leads to a panic on queries.

Related Issue(s)

Fixes #17750

Checklist

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

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 80.55191% with 148 lines in your changes missing coverage. Please review.

Project coverage is 67.42%. Comparing base (2118bc3) to head (4171a5e).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/evalengine/eval_temporal.go	17.72%	65 Missing ⚠️
go/vt/vtgate/evalengine/fn_compare.go	89.03%	25 Missing ⚠️
go/vt/vtgate/evalengine/compiler_asm_push.go	0.00%	21 Missing ⚠️
go/vt/vtgate/evalengine/compiler_asm.go	60.97%	16 Missing ⚠️
go/vt/vtgate/evalengine/compiler.go	44.44%	15 Missing ⚠️
go/vt/vtgate/evalengine/expr_bvar.go	0.00%	3 Missing ⚠️
go/vt/vtgate/evalengine/expr_column.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17826      +/-   ##
==========================================
- Coverage   67.45%   67.42%   -0.03%     
==========================================
  Files        1592     1592              
  Lines      258167   258554     +387     
==========================================
+ Hits       174143   174338     +195     
- Misses      84024    84216     +192     

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[systay]

shouldn't we use skipCollationCheck here?

[dbussink]

@systay This currently doesn't do collations checks anyway, so I don't think it really matters.

[dbussink]

@systay This did remind me that we actually should validate the collation. We still ignore this flag though, since our evalengine and compiler should always match.

The ignore collation flag is for where explicitly don't 100% match MySQL (due to bugs, arguably broken behavior etc).

[systay]

nitpick: another solution would be to use the variable name datetimes in the functions, and keep import named datetime

[systay]

LGTM

[dbussink]

skipCollationCheck is needed because of the issue in MySQL described in mysql/mysql-server#602.