Add warning for poor compression ratio

Adds functionality to emit a warning to the user when the compression
ratio is "bad".

Currently, "bad" is defined to be when size after compression is higher
than after compression.

.unreleased/pr_7756

@@ -0,0 +1 @@
+Implements: #7756 Add warning for poor compression ratio

src/guc.c

@@ -153,6 +153,7 @@ TSDLLEXPORT bool ts_guc_default_hypercore_use_access_method = false;
 bool ts_guc_enable_chunk_skipping = false;
 TSDLLEXPORT bool ts_guc_enable_segmentwise_recompression = true;
 TSDLLEXPORT bool ts_guc_enable_bool_compression = false;
+TSDLLEXPORT bool ts_guc_enable_compression_ratio_warnings = true;
 
 /* Enable of disable columnar scans for columnar-oriented storage engines. If
  * disabled, regular sequence scans will be used instead. */
@@ -761,6 +762,16 @@ _guc_init(void)
 							 NULL,
 							 NULL);
 
+	DefineCustomBoolVariable(MAKE_EXTOPTION("enable_compression_ratio_warnings"),
+							 "Enable warnings for poor compression ratio",
+							 "Enable warnings for poor compression ratio",
+							 &ts_guc_enable_compression_ratio_warnings,
+							 true,
+							 PGC_USERSET,
+							 0,
+							 NULL,
+							 NULL,
+							 NULL);
 	/*
 	 * Define the limit on number of invalidation-based refreshes we allow per
 	 * refresh call. If this limit is exceeded, fall back to a single refresh that

src/guc.h

@@ -70,6 +70,7 @@ extern TSDLLEXPORT bool ts_guc_enable_merge_on_cagg_refresh;
 extern bool ts_guc_enable_chunk_skipping;
 extern TSDLLEXPORT bool ts_guc_enable_segmentwise_recompression;
 extern TSDLLEXPORT bool ts_guc_enable_bool_compression;
+extern TSDLLEXPORT bool ts_guc_enable_compression_ratio_warnings;
 
 #ifdef USE_TELEMETRY
 typedef enum TelemetryLevel

test/postgresql.conf.in

@@ -25,3 +25,4 @@ extra_float_digits=0
 @TELEMETRY_DEFAULT_SETTING@
 
 timescaledb.license='apache'
+timescaledb.enable_compression_ratio_warnings=false

tsl/src/compression/api.c

@@ -544,6 +544,17 @@ compress_chunk_impl(Oid hypertable_relid, Oid chunk_relid)
 		 */
 		ts_chunk_constraints_create(cxt.compress_ht, compress_ht_chunk);
 		ts_trigger_create_all_on_chunk(compress_ht_chunk);
+
+		/* Detect and emit warning if poor compression ratio is found */
+		float compression_ratio = ((float) before_size.total_size / after_size.total_size);
+		float POOR_COMPRESSION_THRESHOLD = 1.0;
+		elog(ts_guc_enable_compression_ratio_warnings &&
+					 compression_ratio < POOR_COMPRESSION_THRESHOLD ?
+				 WARNING :
+				 DEBUG1,
+			 "size before compression: %ld bytes, size after compression: %ld bytes",
+			 before_size.total_size,
+			 after_size.total_size);
 	}
 	else
 	{

tsl/test/postgresql.conf.in

@@ -35,3 +35,4 @@ log_statement='all'
 # It is necessary to have at least 2 more workers than `max_worker_processes`
 # in order to test failures starting bgworkers.
 timescaledb.max_background_workers=26
+timescaledb.enable_compression_ratio_warnings=false