expression: reserve exact space for UUIDs

[dveeden]

What problem does this PR solve?

Issue Number: close #65364

Problem Summary:

result.ReserveString(n) reserves space for a string, but doesn't take into account that UUID(), UUID_V4() and UUID_V7() all create 36-byte text format UUIDs.

What changed and how does it work?

With a t1 table with 36 rows:

select uuid() from t1;

(dlv) b (*builtinUUIDSig).vecEvalString
Breakpoint 1 set at 0x563f173 for github.com/pingcap/tidb/pkg/expression.(*builtinUUIDSig).vecEvalString() ./pkg/expression/builtin_miscellaneous_vec.go:210
(dlv) c

(dlv) p n
36
(dlv) p result
("*github.com/pingcap/tidb/pkg/util/chunk.Column")(0xc015480070)
*github.com/pingcap/tidb/pkg/util/chunk.Column {
	length: 0,
	nullBitmap: []uint8 len: 0, cap: 4, [],
	offsets: []int64 len: 1, cap: 33, [0],
	data: []uint8 len: 0, cap: 256, [],
	elemBuf: []uint8 len: 0, cap: 0, nil,
	avoidReusing: false,}

So we have n=36 for 36 rows. And data has a capacity of 256 before calling result.ReserveStringWithSizeHint(n, UUID_STR_LEN)

(dlv) n
> github.com/pingcap/tidb/pkg/expression.(*builtinUUIDSig).vecEvalString() ./pkg/expression/builtin_miscellaneous_vec.go:213 (PC: 0x563f1d2)
   208:	}
   209:	
   210:	func (b *builtinUUIDSig) vecEvalString(ctx EvalContext, input *chunk.Chunk, result *chunk.Column) error {
   211:		n := input.NumRows()
   212:		result.ReserveStringWithSizeHint(n, UUID_STR_LEN)
=> 213:		var id uuid.UUID
   214:		var err error
   215:		for range n {
   216:			id, err = uuid.NewUUID()
   217:			if err != nil {
   218:				return err
(dlv) p result
("*github.com/pingcap/tidb/pkg/util/chunk.Column")(0xc015480070)
*github.com/pingcap/tidb/pkg/util/chunk.Column {
	length: 0,
	nullBitmap: []uint8 len: 0, cap: 5, [],
	offsets: []int64 len: 1, cap: 37, [0],
	data: []uint8 len: 0, cap: 1296, [],
	elemBuf: []uint8 len: 0, cap: 0, nil,
	avoidReusing: false,}
(dlv) p 36*36
1296

So after the call to ReserveStringWithSizeHint() (with (36, 36) as arguments for 36 rows of 36 bytes/chars) we have a data with a capacity of 1296 (= 36*36).

Now let's set a breakpoint at the line where we return form this function.

(dlv) b builtin_miscellaneous_vec.go:222
Breakpoint 2 set at 0x563f2dd for github.com/pingcap/tidb/pkg/expression.(*builtinUUIDSig).vecEvalString() ./pkg/expression/builtin_miscellaneous_vec.go:222
(dlv) c
[2025/12/31 09:44:25.576 +01:00] [ERROR] [syncer.go:314] ["reload schema in loop failed"] [error="[tikv:9001]PD server timeout: start timestamp may fall behind safe point"]
> [Breakpoint 2] github.com/pingcap/tidb/pkg/expression.(*builtinUUIDSig).vecEvalString() ./pkg/expression/builtin_miscellaneous_vec.go:222 (hits goroutine(2075):1 total:1) (PC: 0x563f2dd)
Warning: listing may not match stale executable
   217:			if err != nil {
   218:				return err
   219:			}
   220:			result.AppendString(id.String())
   221:		}
=> 222:		return nil
   223:	}
   224:	
   225:	func (b *builtinUUIDv4Sig) vectorized() bool {
   226:		return true
   227:	}
(dlv) p result
("*github.com/pingcap/tidb/pkg/util/chunk.Column")(0xc015480070)
*github.com/pingcap/tidb/pkg/util/chunk.Column {
	length: 36,
	nullBitmap: []uint8 len: 5, cap: 5, [255,255,255,255,15],
	offsets: []int64 len: 37, cap: 37, [0,36,72,108,144,180,216,252,288,324,360,396,432,468,504,540,576,612,648,684,720,756,792,828,864,900,936,972,1008,1044,1080,1116,1152,1188,1224,1260,1296],
	data: []uint8 len: 1296, cap: 1296, [101,57,52,98,100,52,101,97,45,101,54,50,52,45,49,49,102,48,45,98,99,99,97,45,50,50,50,50,50,100,51,52,100,52,49,49,101,57,52,98,101,51,54,97,45,101,54,50,52,45,49,49,102,48,45,98,99,99,97,45,50,50,50,50,...+1232 more],
	elemBuf: []uint8 len: 0, cap: 0, nil,
	avoidReusing: false,}
(dlv) 

So after getting 36 UUID's we do see a data with a capacity of 1296 and a length of 1296.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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[dveeden]

/ok-to-test

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.5455%. Comparing base (2114fbf) to head (9976715).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #65365        +/-   ##
================================================
- Coverage   78.3390%   76.5455%   -1.7936%     
================================================
  Files          1940       1953        +13     
  Lines        535314     539223      +3909     
================================================
- Hits         419360     412751      -6609     
- Misses       114433     125663     +11230     
+ Partials       1521        809       -712     

Flag	Coverage Δ
integration	41.5622% <100.0000%> (-6.6225%)	⬇️
unit	76.7752% <0.0000%> (+0.2882%)	⬆️

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Components	Coverage Δ
dumpling	56.6191% <ø> (ø)
parser	∅ <ø> (∅)
br	49.5181% <ø> (-15.3681%)	⬇️

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[windtalker]

lgtm

[zanmato1984]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: windtalker, zanmato1984

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-01-07 02:59:43.007296904 +0000 UTC m=+757538.825605336: ☑️ agreed by windtalker.
• 2026-01-07 06:27:07.11475104 +0000 UTC m=+769982.933059471: ☑️ agreed by zanmato1984.

[dveeden]

/retest

[dveeden]

/retest

[dveeden]

/retest