Add function to show compression information

Add a function that can be used on a compressed data value to show
some metadata information, such as the compression algorithm used and
the presence of any null values.

.unreleased/pr_7126

@@ -0,0 +1 @@
+Implements: #7126 Add functions to show compression info

sql/pre_install/types.functions.sql

@@ -34,6 +34,11 @@ CREATE OR REPLACE FUNCTION _timescaledb_functions.compressed_data_recv(internal)
    AS '@MODULE_PATHNAME@', 'ts_compressed_data_recv'
    LANGUAGE C IMMUTABLE STRICT;
 
+CREATE OR REPLACE FUNCTION _timescaledb_functions.compressed_data_info(_timescaledb_internal.compressed_data)
+    RETURNS TABLE (algorithm name, has_nulls bool)
+    LANGUAGE C STRICT IMMUTABLE
+    AS '@MODULE_PATHNAME@', 'ts_compressed_data_info';
+
 CREATE OR REPLACE FUNCTION _timescaledb_functions.dimension_info_in(cstring)
     RETURNS _timescaledb_internal.dimension_info
     LANGUAGE C STRICT IMMUTABLE

sql/updates/latest-dev.sql

@@ -65,3 +65,8 @@ SELECT pg_catalog.pg_extension_config_dump(pg_get_serial_sequence('_timescaledb_
 
 GRANT SELECT ON _timescaledb_catalog.chunk_column_stats TO PUBLIC;
 GRANT SELECT ON _timescaledb_catalog.chunk_column_stats_id_seq TO PUBLIC;
+
+CREATE FUNCTION _timescaledb_functions.compressed_data_info(_timescaledb_internal.compressed_data)
+    RETURNS TABLE (algorithm name, has_nulls bool)
+    LANGUAGE C STRICT IMMUTABLE
+    AS '@MODULE_PATHNAME@', 'ts_compressed_data_info';

sql/updates/reverse-dev.sql

@@ -5,3 +5,4 @@ DROP FUNCTION IF EXISTS @[email protected]_column_stats(REGCLASS, NAME, BOOLEAN
 ALTER EXTENSION timescaledb DROP TABLE _timescaledb_catalog.chunk_column_stats;
 ALTER EXTENSION timescaledb DROP SEQUENCE _timescaledb_catalog.chunk_column_stats_id_seq;
 DROP TABLE IF EXISTS _timescaledb_catalog.chunk_column_stats;
+DROP FUNCTION _timescaledb_functions.compressed_data_info(_timescaledb_internal.compressed_data);

src/cross_module_fn.c

@@ -66,6 +66,7 @@ CROSSMODULE_WRAPPER(compressed_data_send);
 CROSSMODULE_WRAPPER(compressed_data_recv);
 CROSSMODULE_WRAPPER(compressed_data_in);
 CROSSMODULE_WRAPPER(compressed_data_out);
+CROSSMODULE_WRAPPER(compressed_data_info);
 CROSSMODULE_WRAPPER(deltadelta_compressor_append);
 CROSSMODULE_WRAPPER(deltadelta_compressor_finish);
 CROSSMODULE_WRAPPER(gorilla_compressor_append);

src/cross_module_fn.h

@@ -118,6 +118,7 @@ typedef struct CrossModuleFunctions
 	PGFunction compressed_data_recv;
 	PGFunction compressed_data_in;
 	PGFunction compressed_data_out;
+	PGFunction compressed_data_info;
 	bool (*process_compress_table)(AlterTableCmd *cmd, Hypertable *ht,
 								   WithClauseResult *with_clause_options);
 	void (*process_altertable_cmd)(Hypertable *ht, const AlterTableCmd *cmd);

tsl/src/compression/algorithms/array.c

@@ -40,6 +40,13 @@ typedef struct ArrayCompressed
 	uint64 alignment_sentinel[FLEXIBLE_ARRAY_MEMBER];
 } ArrayCompressed;
 
+bool
+array_compressed_has_nulls(const CompressedDataHeader *header)
+{
+	const ArrayCompressed *ac = (const ArrayCompressed *) header;
+	return ac->has_nulls;
+}
+
 static void
 pg_attribute_unused() assertions(void)
 {

tsl/src/compression/algorithms/array.h

@@ -26,6 +26,7 @@ typedef struct ArrayDecompressionIterator ArrayDecompressionIterator;
 
 extern const Compressor array_compressor;
 
+extern bool array_compressed_has_nulls(const CompressedDataHeader *header);
 extern Compressor *array_compressor_for_type(Oid element_type);
 extern ArrayCompressor *array_compressor_alloc(Oid type_to_compress);
 extern void array_compressor_append_null(ArrayCompressor *compressor);

tsl/src/compression/algorithms/deltadelta.c

@@ -12,6 +12,7 @@
 #include <common/base64.h>
 #include <funcapi.h>
 #include <lib/stringinfo.h>
+#include <stdbool.h>
 #include <utils/builtins.h>
 #include <utils/date.h>
 #include <utils/lsyscache.h>
@@ -76,6 +77,13 @@ typedef struct ExtendedCompressor
 	DeltaDeltaCompressor *internal;
 } ExtendedCompressor;
 
+bool
+deltadelta_compressed_has_nulls(const CompressedDataHeader *header)
+{
+	const DeltaDeltaCompressed *ddc = (const DeltaDeltaCompressed *) header;
+	return ddc->has_nulls;
+}
+
 static void
 deltadelta_compressor_append_bool(Compressor *compressor, Datum val)
 {

tsl/src/compression/algorithms/deltadelta.h

@@ -27,6 +27,7 @@ typedef struct DeltaDeltaCompressor DeltaDeltaCompressor;
 typedef struct DeltaDeltaCompressed DeltaDeltaCompressed;
 typedef struct DeltaDeltaDecompressionIterator DeltaDeltaDecompressionIterator;
 
+extern bool deltadelta_compressed_has_nulls(const CompressedDataHeader *header);
 extern Compressor *delta_delta_compressor_for_type(Oid element_type);
 extern DeltaDeltaCompressor *delta_delta_compressor_alloc(void);
 extern void delta_delta_compressor_append_null(DeltaDeltaCompressor *compressor);

tsl/src/compression/algorithms/dictionary.c

@@ -48,6 +48,13 @@ typedef struct DictionaryCompressed
 	uint64 alignment_sentinel[FLEXIBLE_ARRAY_MEMBER];
 } DictionaryCompressed;
 
+bool
+dictionary_compressed_has_nulls(const CompressedDataHeader *header)
+{
+	const DictionaryCompressed *dc = (const DictionaryCompressed *) header;
+	return dc->has_nulls;
+}
+
 static void
 pg_attribute_unused() assertions(void)
 {

tsl/src/compression/algorithms/dictionary.h

@@ -23,6 +23,7 @@ typedef struct DictionaryCompressor DictionaryCompressor;
 typedef struct DictionaryCompressed DictionaryCompressed;
 typedef struct DictionaryDecompressionIterator DictionaryDecompressionIterator;
 
+extern bool dictionary_compressed_has_nulls(const CompressedDataHeader *header);
 extern Compressor *dictionary_compressor_for_type(Oid element_type);
 extern DictionaryCompressor *dictionary_compressor_alloc(Oid type_to_compress);
 extern void dictionary_compressor_append_null(DictionaryCompressor *compressor);

tsl/src/compression/algorithms/gorilla.c

@@ -62,6 +62,13 @@ typedef struct CompressedGorillaData
 	Simple8bRleSerialized *nulls; /* NULL if no nulls */
 } CompressedGorillaData;
 
+bool
+gorilla_compressed_has_nulls(const CompressedDataHeader *header)
+{
+	const GorillaCompressed *gc = (const GorillaCompressed *) header;
+	return gc->has_nulls;
+}
+
 static void
 pg_attribute_unused() assertions(void)
 {

tsl/src/compression/algorithms/gorilla.h

@@ -71,6 +71,7 @@ typedef struct GorillaCompressor GorillaCompressor;
 typedef struct GorillaCompressed GorillaCompressed;
 typedef struct GorillaDecompressionIterator GorillaDecompressionIterator;
 
+extern bool gorilla_compressed_has_nulls(const CompressedDataHeader *header);
 extern Compressor *gorilla_compressor_for_type(Oid element_type);
 
 extern GorillaCompressor *gorilla_compressor_alloc(void);

tsl/src/compression/compression.c

@@ -46,6 +46,18 @@ static const CompressionAlgorithmDefinition definitions[_END_COMPRESSION_ALGORIT
 	[COMPRESSION_ALGORITHM_DELTADELTA] = DELTA_DELTA_ALGORITHM_DEFINITION,
 };
 
+static const char *compression_algorithm_name[] = {
+	[_INVALID_COMPRESSION_ALGORITHM] = "INVALID",	   [COMPRESSION_ALGORITHM_ARRAY] = "ARRAY",
+	[COMPRESSION_ALGORITHM_DICTIONARY] = "DICTIONARY", [COMPRESSION_ALGORITHM_GORILLA] = "GORILLA",
+	[COMPRESSION_ALGORITHM_DELTADELTA] = "DELTADELTA",
+};
+
+const char *
+compression_get_algorithm_name(CompressionAlgorithm alg)
+{
+	return compression_algorithm_name[alg];
+}
+
 static Compressor *
 compressor_for_type(Oid type)
 {
@@ -1934,6 +1946,59 @@ tsl_compressed_data_out(PG_FUNCTION_ARGS)
 	PG_RETURN_CSTRING(encoded);
 }
 
+/* create_hypertable record attribute numbers */
+enum Anum_compressed_info
+{
+	Anum_compressed_info_algorithm = 1,
+	Anum_compressed_info_has_nulls,
+	_Anum_compressed_info_max,
+};
+
+#define Natts_compressed_info (_Anum_compressed_info_max - 1)
+
+extern Datum
+tsl_compressed_data_info(PG_FUNCTION_ARGS)
+{
+	const CompressedDataHeader *header = (CompressedDataHeader *) PG_GETARG_VARLENA_P(0);
+	TupleDesc tupdesc;
+	HeapTuple tuple;
+	bool has_nulls = false;
+
+	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("function returning record called in "
+						"context that cannot accept type record")));
+
+	switch (header->compression_algorithm)
+	{
+		case COMPRESSION_ALGORITHM_GORILLA:
+			has_nulls = gorilla_compressed_has_nulls(header);
+			break;
+		case COMPRESSION_ALGORITHM_DICTIONARY:
+			has_nulls = dictionary_compressed_has_nulls(header);
+			break;
+		case COMPRESSION_ALGORITHM_DELTADELTA:
+			has_nulls = deltadelta_compressed_has_nulls(header);
+			break;
+		case COMPRESSION_ALGORITHM_ARRAY:
+			has_nulls = array_compressed_has_nulls(header);
+			break;
+	}
+
+	tupdesc = BlessTupleDesc(tupdesc);
+
+	Datum values[Natts_compressed_info];
+	bool nulls[Natts_compressed_info] = { false };
+
+	values[AttrNumberGetAttrOffset(Anum_compressed_info_algorithm)] =
+		CStringGetDatum(compression_get_algorithm_name(header->compression_algorithm));
+	values[AttrNumberGetAttrOffset(Anum_compressed_info_has_nulls)] = BoolGetDatum(has_nulls);
+	tuple = heap_form_tuple(tupdesc, values, nulls);
+
+	return HeapTupleGetDatum(tuple);
+}
+
 extern CompressionStorage
 compression_get_toast_storage(CompressionAlgorithm algorithm)
 {

tsl/src/compression/compression.h

@@ -296,6 +296,7 @@ extern Datum tsl_compressed_data_send(PG_FUNCTION_ARGS);
 extern Datum tsl_compressed_data_recv(PG_FUNCTION_ARGS);
 extern Datum tsl_compressed_data_in(PG_FUNCTION_ARGS);
 extern Datum tsl_compressed_data_out(PG_FUNCTION_ARGS);
+extern Datum tsl_compressed_data_info(PG_FUNCTION_ARGS);
 
 static void
 pg_attribute_unused() assert_num_compression_algorithms_sane(void)
@@ -320,6 +321,7 @@ pg_attribute_unused() assert_num_compression_algorithms_sane(void)
 					 "number of algorithms have changed, the asserts should be updated");
 }
 
+extern const char *compression_get_algorithm_name(CompressionAlgorithm alg);
 extern CompressionStorage compression_get_toast_storage(CompressionAlgorithm algo);
 extern CompressionAlgorithm compression_get_default_algorithm(Oid typeoid);
 

tsl/src/init.c

@@ -144,6 +144,7 @@ CrossModuleFunctions tsl_cm_functions = {
 	.compressed_data_recv = tsl_compressed_data_recv,
 	.compressed_data_in = tsl_compressed_data_in,
 	.compressed_data_out = tsl_compressed_data_out,
+	.compressed_data_info = tsl_compressed_data_info,
 	.deltadelta_compressor_append = tsl_deltadelta_compressor_append,
 	.deltadelta_compressor_finish = tsl_deltadelta_compressor_finish,
 	.gorilla_compressor_append = tsl_gorilla_compressor_append,

tsl/test/shared/expected/extension.out

@@ -51,6 +51,7 @@ ORDER BY pronamespace::regnamespace::text COLLATE "C", p.oid::regprocedure::text
  _timescaledb_functions.chunks_local_size(name,name)
  _timescaledb_functions.compressed_chunk_local_stats(name,name)
  _timescaledb_functions.compressed_data_in(cstring)
+ _timescaledb_functions.compressed_data_info(_timescaledb_internal.compressed_data)
  _timescaledb_functions.compressed_data_out(_timescaledb_internal.compressed_data)
  _timescaledb_functions.compressed_data_recv(internal)
  _timescaledb_functions.compressed_data_send(_timescaledb_internal.compressed_data)