Remove the dead column cache (#303)

CHANGELOG.md

@@ -130,6 +130,16 @@ unreleased. For the forward-looking plan see
 - Lost delete marks under concurrent same-chunk-group deletes.
 - Relation-reference leak in parallel `CREATE INDEX`.
 
+### Removed
+
+- The decompressed-chunk cache, and the `pgcolumnar.enable_column_cache` and
+  `pgcolumnar.column_cache_size` settings with it. Its only entry point had lost
+  its caller when the earlier on-disk format was removed, so the cache had done
+  nothing since. Two settings and four passages of documentation described a
+  feature that did not run. A `postgresql.conf` that sets either parameter must
+  drop the line. The implementation is in the git history if the performance case
+  is made again against the current reader.
+
 ### Changed
 
 - FSST string encoding is now kept only when it reduces the compressed chunk by

Makefile

@@ -15,7 +15,6 @@ OBJS = \
 	src/columnar_reader.o \
 	src/columnar_delete_vector.o \
 	src/columnar_customscan.o \
-	src/columnar_cache.o \
 	src/columnar_vector.o \
 	src/columnar_vacuum.o \
 	src/columnar_unique.o \

docs/ARCHITECTURE.md

@@ -75,7 +75,7 @@ storage, bulk and single insert, sequential scan open/next/close, delete and
 update (through the row mask), fetch a row by item pointer, size estimation, and
 truncate. It also holds `_PG_init`, which registers every `pgcolumnar.*` GUC (the
 compression codec and level, the row-group and vector row limits, and the qual
-pushdown, custom scan, vectorized aggregate, and column cache toggles), the
+pushdown, custom scan and vectorized aggregate toggles), the
 pre-commit hook that flushes pending writes, and the object-access hook that
 removes a table's metadata rows when the table is dropped.
 
@@ -208,15 +208,6 @@ answered from the zone-map metadata, falling back to a scan-and-fold when the
 group has deletes. It is chosen only when every aggregate, column type, and
 clause is supported; anything else falls back to the scalar plan.
 
-### columnar_cache.c
-The optional decompressed-chunk cache, off by default behind
-`pgcolumnar.enable_column_cache` and bounded by `pgcolumnar.column_cache_size`
-megabytes. It is a backend-local, LRU-bounded cache of decompressed value
-streams keyed by storage id and absolute logical offset. It returns a fresh copy
-to the caller so eviction is always safe, and it is flushed on any relcache
-invalidation so a truncate offset reuse or a vacuum storage swap can never serve
-a stale buffer. It only avoids repeated decompression; it never changes results.
-
 ### columnar_vacuum.c
 Compaction, statistics, and storage-id lookup. `pgcolumnar.vacuum` materializes a
 relation's live rows (the reader skips row-mask-deleted rows), swaps the
@@ -373,7 +364,7 @@ Scan:
 2. The reader (`columnar_reader`) goes through the row groups. It uses the zone
    maps in `columnar_metadata` to skip groups and vectors.
 3. The reader decodes the projected chunks through `columnar_encoding` and
-   `columnar_compression`, and can use `columnar_cache`.
+   `columnar_compression`.
 4. The reader applies the delete vector (`columnar_delete_vector`) and returns
    the rows one at a time.
 5. The executor applies the full qual again, as a filter.

docs/administration.md

@@ -205,11 +205,9 @@ specific reason to change it.
 
 ## Column cache
 
-`pgcolumnar.enable_column_cache` keeps chunk groups after decompression. Other
-reads can then use them again. `pgcolumnar.column_cache_size` sets the size, and
-the default is 200 MB. The cache is off by default. Enable it if you scan the
-same recent data more than one time. Set the size to the size of the working
-set.
+There is no cache of decompressed chunk groups. A cache existed in an earlier
+build, but its only entry point lost its caller and the code did nothing. It was
+removed in #303 rather than left as a setting that changes nothing.
 
 ## Backup and restore
 

docs/configuration.md

@@ -69,8 +69,6 @@ disk. It never changes the values that a table returns.
 
 | Setting | Type | Default | Description |
 | --- | --- | --- | --- |
-| `pgcolumnar.enable_column_cache` | boolean | `off` | Cache decompressed chunk groups so they can be reused across reads. |
-| `pgcolumnar.column_cache_size` | integer (MB) | `200` | Size of the decompressed-chunk cache. Applies when the column cache is enabled. Range 1 to INT_MAX. |
 
 ### Maintenance and disk reclaim
 

docs/testing.md

@@ -10,7 +10,7 @@ test/phase2.sh  /path/to/pg_config   # compression, projection, min/max skip, fi
 test/phase3.sh  /path/to/pg_config   # delete, update, MVCC, savepoints, temp tables
 test/phase4.sh  /path/to/pg_config   # btree/hash indexes, constraints, conversion
 test/phase5.sh  /path/to/pg_config   # custom scan, pushdown, options, vacuum
-test/phase6.sh  /path/to/pg_config   # aggregate correctness and the column cache
+test/phase6.sh  /path/to/pg_config   # aggregate correctness
 test/audit.sh   /path/to/pg_config   # regression tests for audited defects
 test/concurrency.sh      /path/to/pg_config  # concurrent same-chunk-group deletes
 test/unique_conc.sh      /path/to/pg_config  # concurrent same-unique-key inserts

src/columnar.h

@@ -173,11 +173,9 @@ extern bool columnar_enable_bloom_filter;	/* bloom equality skipping (I7) */
 
 /* Phase 6 GUCs (spec 8.3) */
 extern bool columnar_enable_vectorization;	/* vectorized aggregate path */
-extern bool columnar_enable_column_cache;	/* decompressed-chunk cache */
 extern bool columnar_enable_read_stream;	/* stream/prefetch block reads (PG17+) */
 extern bool columnar_enable_index_only_scan;	/* allow index-only scans (gap 28) */
 extern bool columnar_enable_projection_scan;	/* scan a covering projection (gap 26) */
-extern int columnar_column_cache_size;		/* cache budget in megabytes */
 
 /* issue #5: concurrent unique-key insert serialization */
 extern bool columnar_enable_unique_lock;	/* serialize same-key inserters */
@@ -741,20 +739,6 @@ extern char *ColumnarDecompressValueStream(const char *comp, uint32 compLen,
 										   int compressionType, uint32 rawLen,
 										   MemoryContext targetContext);
 
-/* -------------------------------------------------------------------------
- * decompressed-chunk cache (columnar_cache.c, spec 8.3, 9)
- *
- * An optional, backend-local cache of decompressed value streams, keyed by the
- * relation's storage id and the stream's absolute logical offset (both stable
- * and never reused within a storage id, except across a truncate, which fires a
- * relcache invalidation that flushes the whole cache). Off by default; when on
- * it only avoids repeated decompression and never changes results.
- * ------------------------------------------------------------------------- */
-extern void ColumnarCacheInit(void);
-extern char *ColumnarGetDecompressedStream(uint64 storageId, uint64 absOffset,
-										   const char *comp, uint32 compLen,
-										   int compressionType, uint32 rawLen,
-										   MemoryContext targetContext);
 
 /* -------------------------------------------------------------------------
  * concurrent unique-key insert serialization (columnar_unique.c, issue #5)