Release v2.4.0 - Expression Index Support and Bug Fixes · CrystallineCore/Biscuit

Version 2.4.0 — Donut

Expression index support: Biscuit now correctly evaluates arbitrary index key expressions during index builds, enabling indexes such as:
```
CREATE INDEX idx ON table USING biscuit (lower(column_1), (column_2::text));
```
Multi-version build support (PG 16, 17, 18, 19beta1): Biscuit can now be compiled and installed against PostgreSQL 16 and 17, in addition to the already supported PG 18 and PG 19 Beta. All version-specific API differences are handled at compile time via #if PG_VERSION_NUM guards.

Multi-column parallel scan returned duplicate rows: In the multi-column fallback scan path, every Gather participant was calling biscuit_collect_sorted_tids_single() unconditionally, causing each worker to return the full TID set and the Gather node to assemble N× the expected rows. The call site now mirrors the single-column path by resolving the shared-memory parallel scan descriptor and dispatching through biscuit_collect_sorted_tids_parallel(), so each participant claims a disjoint slice of the pre-partitioned TID array.
biscuit_operators view no longer breaks when additional operator classes are added: The view previously filtered on a hardcoded opfname = 'biscuit_text_ops'. It now joins through pg_am and filters on am.amname = 'biscuit', staying correct without edits if new opclasses or opfamilies are later added. The view also surfaces the opfamily name per row.

Parallel scan callbacks are conditionally compiled for PG 18+: amcanparallel, amestimateparallelscan, aminitparallelscan, and amparallelrescan are only registered when PG_VERSION_NUM >= 180000. On PG 16 and 17 the parallel fields are set to false / NULL.
Cross-version compatibility macros added to biscuit_common.h:
- BISCUIT_PARALLEL_AM_OFFSET(ps) abstracts the rename of ps_offset → ps_offset_am in PG 18.
- BISCUIT_COUNT_INDEX_SEARCH(scan) abstracts the index search counter, which moved from xs_numIndexSearches (PG 17) to scan->instrument->nsearches (PG 18+) and did not exist in PG 16.
- biscuit_estimateparallelscan is declared with the correct signature for each major version (void on PG 16, int nkeys, int norderbys on PG 17, Relation indexRelation, int nworkers, int nchunks on PG 18+).
Version string updated to 2.4.0 - Donut.

CHAR(n) / bpchar native operator class is not yet available. PostgreSQL defines LIKE/ILIKE operators only over (text, text), so a dedicated biscuit_bpchar_ops operating directly on padded bpchar values would require new C-level operator implementations. As a supported workaround, CHAR(n) columns can be indexed today via an expression index on the text cast:
```
CREATE INDEX idx ON table USING biscuit ((char_col::text));
```
This is documented in biscuit.sql and reflected in the updated biscuit_operators view comment.