Fix incorrect or missed pruning when startsWith, LIKE, NOT LIKE used with FixedString column

[nihalzp]

Given,

CREATE TABLE test (fixed_string_col FixedString(40), text String) ENGINE = MergeTree ORDER BY fixed_string_col SETTINGS index_granularity = 1;

INSERT INTO test VALUES ('11','plain-11'),('110','plain-110'),('11z','plain-11z'),('12','plain-12'),('21','plain-21'),('999','plain-999');

Wrong answer:

SELECT count()
FROM test
WHERE startsWith(fixed_string_col, '11'); -- returns 1 but should be 3

SELECT count()
FROM test
WHERE fixed_string_col LIKE '11%'; -- returns 1 but should be 3

SELECT text
FROM test
WHERE fixed_string_col NOT LIKE '99%'; -- Read more than needed because `LIKE` internval was incorrectly small so `NOT LIKE` became larger

Addtionally, FixedString to String cast function monotonicity added. As a result, we prune for cases like these:

SELECT text
FROM test
WHERE startsWith(CAST(fixed_string_col, 'String'), '11');

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes into CHANGELOG.md):

Fix incorrect or less pruning when startsWith, LIKE, NOT LIKE used with FixedString column. Additionally, FixedString to String cast function can now prune granules when wrapped around key column. Closes #98940.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

---

Note

Medium Risk
Changes predicate-to-range conversion in KeyCondition and function monotonicity metadata, which can affect query correctness and index pruning behavior for MergeTree scans.

Overview
Fixes incorrect/overly-narrow primary-key range construction when startsWith, LIKE/NOT LIKE, or match are applied to FixedString key columns by preserving string literals instead of converting them to padded FixedString(N) during key-condition analysis.

Marks toString(FixedString) (including LowCardinality(FixedString)) as always strict monotonic, allowing MergeTree pruning when casts wrap the key column. Adds unit tests for the new monotonicity and stateless query tests covering the FixedString prefix/pattern pruning regressions.

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Workflow [PR], commit [30ae7ae]

Summary: ✅

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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Branches	75.20%	76.30%	+1.10%

PR changed lines: PR changed-lines coverage: 97.83% (45/46)
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[SmitaRKulkarni]

LGTM