Remove CAgg watermark cache

Since #6325 we constify the watermark value of a CAgg during planning
time. Therefore, the planner calls the watermark function only once.
This PR removes the old code to cache the watermark value and speed up
multiple calls of the watermark function.

src/ts_catalog/continuous_aggs_watermark.c

@@ -21,53 +21,6 @@
 #include "ts_catalog/continuous_aggs_watermark.h"
 #include "hypertable.h"
 
-typedef struct ContinuousAggregateWatermark
-{
-	int32 mat_hypertable_id;
-	MemoryContext mctx;
-	MemoryContextCallback cb;
-	CommandId cid;
-	int64 value;
-} ContinuousAggregateWatermark;
-
-/*
- * Cache the watermark in the current transaction for better performance
- * (by avoiding repeated max bucket calculations). The watermark will be
- * reset at the end of the transaction, when the watermark function's input
- * argument (materialized hypertable ID) changes, or when a new command is
- * executed.
- *
- * One could potentially create a hashtable of watermarks keyed on materialized
- * hypertable ID, but this is left as a future optimization since it doesn't
- * seem to be common case that multiple continuous aggregates exist in the same
- * query. Besides, the planner can constify calls to the watermark function
- * during planning since the function is STABLE. Therefore, this is only a
- * fallback if the planner needs to constify it many times (e.g., if used as
- * an index condition on many chunks).
- */
-static ContinuousAggregateWatermark *cagg_watermark_cache = NULL;
-
-/*
- * Callback handler to reset the watermark after the transaction ends. This is
- * triggered by the deletion of the associated memory context.
- */
-static void
-cagg_watermark_reset(void *arg)
-{
-	cagg_watermark_cache = NULL;
-}
-
-/*
- * ContinuousAggregateWatermark is valid for the duration of one command execution on the same
- * materialized hypertable.
- */
-static bool
-cagg_watermark_is_valid(const ContinuousAggregateWatermark *cagg_watermark, int32 mat_hypertable_id)
-{
-	return cagg_watermark != NULL && cagg_watermark->mat_hypertable_id == mat_hypertable_id &&
-		   cagg_watermark->cid == GetCurrentCommandId(false);
-}
-
 static void
 cagg_watermark_init_scan_by_mat_hypertable_id(ScanIterator *iterator, const int32 mat_hypertable_id)
 {
@@ -128,37 +81,6 @@ ts_cagg_watermark_get(int32 hypertable_id)
 	return DatumGetInt64(watermark);
 }
 
-static ContinuousAggregateWatermark *
-cagg_watermark_create(const ContinuousAgg *cagg, MemoryContext top_mctx)
-{
-	Hypertable *ht;
-	ContinuousAggregateWatermark *watermark;
-	MemoryContext mctx = AllocSetContextCreate(top_mctx,
-											   "ContinuousAggregateWatermark function",
-											   ALLOCSET_DEFAULT_SIZES);
-
-	watermark = MemoryContextAllocZero(mctx, sizeof(ContinuousAggregateWatermark));
-	watermark->mctx = mctx;
-	watermark->mat_hypertable_id = cagg->data.mat_hypertable_id;
-	watermark->cid = GetCurrentCommandId(false);
-	watermark->cb.func = cagg_watermark_reset;
-	MemoryContextRegisterResetCallback(mctx, &watermark->cb);
-
-	/* Hypertable associated to the Continuous Aggregate */
-	ht = ts_hypertable_get_by_id(cagg->data.mat_hypertable_id);
-
-	if (NULL == ht)
-		ereport(ERROR,
-				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("invalid materialization hypertable ID: %d",
-						cagg->data.mat_hypertable_id)));
-
-	/* Get the stored watermark */
-	watermark->value = ts_cagg_watermark_get(cagg->data.mat_hypertable_id);
-
-	return watermark;
-}
-
 TS_FUNCTION_INFO_V1(ts_continuous_agg_watermark);
 
 /*
@@ -183,14 +105,6 @@ ts_continuous_agg_watermark(PG_FUNCTION_ARGS)
 	ContinuousAgg *cagg;
 	AclResult aclresult;
 
-	if (NULL != cagg_watermark_cache)
-	{
-		if (cagg_watermark_is_valid(cagg_watermark_cache, mat_hypertable_id))
-			PG_RETURN_INT64(cagg_watermark_cache->value);
-
-		MemoryContextDelete(cagg_watermark_cache->mctx);
-	}
-
 	cagg = ts_continuous_agg_find_by_mat_hypertable_id(mat_hypertable_id, false);
 
 	/*
@@ -199,9 +113,10 @@ ts_continuous_agg_watermark(PG_FUNCTION_ARGS)
 	 */
 	aclresult = pg_class_aclcheck(cagg->relid, GetUserId(), ACL_SELECT);
 	aclcheck_error(aclresult, OBJECT_MATVIEW, get_rel_name(cagg->relid));
-	cagg_watermark_cache = cagg_watermark_create(cagg, TopTransactionContext);
 
-	PG_RETURN_INT64(cagg_watermark_cache->value);
+	int64 watermark = ts_cagg_watermark_get(cagg->data.mat_hypertable_id);
+
+	PG_RETURN_INT64(watermark);
 }
 
 static int64