CONFIGURATION.md

Configuration reference

All settings are PostgreSQL GUCs and follow the usual scoping rules (SET, ALTER SYSTEM, postgresql.conf).

Parallelism

GUC	Default	Context	Description
pg_deltax.parallel_workers	0	userset	Number of internal Rust worker threads used by parallel aggregation inside a custom scan. 0 = auto (CPU count, capped at 16); 1 = single-threaded; 2..=64 explicit.
pg_deltax.max_parallel_workers_per_scan	-1	userset	Cap on PG parallel workers for DeltaXAppend partial paths. -1 follows max_parallel_workers_per_gather; 0 disables the partial-path variant (scans run serially); 1..=64 caps explicitly.
pg_deltax.parallel_regex	on	userset	When ON, compatible REGEXP_REPLACE(...) patterns used inside GROUP BY use the Rust regex crate so they can run thread-safe in parallel workers.

Blob cache (shared memory)

GUC	Default	Context	Description
pg_deltax.blob_cache_mb	-1 (auto)	postmaster	Size of the process-shared blob cache, in MiB. -1 = auto (25% of physical RAM, clamped to [256, 4096]); 0 = cache disabled; N > 0 = explicit MiB (up to 32768). Restart required — the shmem reservation is captured at postmaster start. See dev/docs/BLOB_CACHE.md.
pg_deltax.blob_cache_shards	64	postmaster	Number of shards (power of two, 1..=1024) in the blob cache. Each shard owns an LWLock + LRU list; more shards reduce contention under high concurrency, fewer save shmem overhead. Restart required.

Optimization toggles

GUC	Default	Context	Description
pg_deltax.bloom_filters	on	userset	Build per-segment bloom filters during compression for equality / IN predicate pushdown. Size is proportional to column cardinality (~2–5% storage overhead). Turning off applies to new compressions only.
pg_deltax.disable_meta_agg_fastpath	off	userset	When ON, DeltaXCount / DeltaXMinMax fast paths are skipped for queries with WHERE clauses; those queries fall through to the generic DeltaXAgg path instead. Used for A/B correctness comparisons.
pg_deltax.disable_parallel_agg	off	userset	When ON, the partial+Gather+FinalAgg path for DeltaXAgg is disabled and the planner only sees the complete CustomScan DeltaXAgg. Escape hatch for bisecting suspected regressions on the partial path; internal-Rust parallelism still runs.
pg_deltax.json_extract_mode	none	userset	How COPY ... WITH (FORMAT deltax_compress) extracts JSON paths into extra columnar columns. none disables extraction and the planner-side rewrite; fields uses the path list configured in deltax_enable_compression(... json_extract => ...); all is reserved for auto-discovery (not yet implemented).
pg_deltax.use_lz4	on	userset	Declare internal columnar-blob companion columns (_blobs._data, _blooms._data, _text_lengths._data, _valbitmap._bits) with BYTEA COMPRESSION lz4. The columnar compression itself happens in Rust regardless; this attribute only controls the Postgres TOAST pass on those already-compressed bytes. Default ON. If the running PG was not built with --with-lz4 the attribute is omitted automatically (and deltax_enable_compression emits a one-shot WARNING per backend) so CREATE TABLE doesn't fail. Set OFF explicitly to suppress the attribute on an lz4-capable build. Without lz4 the on-disk size is somewhat larger and cold-cache reads slower.

Testing

GUC	Default	Context	Description
pg_deltax.mock_now	(empty)	suset	Override current time with a timestamptz literal. Empty string = use real wall-clock time. Used by the test suite to drive deterministic time-based behavior in the background worker and partition-creation paths.