Release memory allocated by dependency_degree

tvondra · tvondra · commit c0386f403a83 · 2021-09-23T18:34:01.000+02:00
Calculating degree of a functional dependency may allocate a lot of memory - we have released mot of the explicitly allocated memory, but e.g. detoasted varlena values were left behind. That may be an issue, because we consider a lot of dependencies (all combinations), and the detoasting may happen for each one again. Fixed by calling dependency_degree() in a dedicated context, and resetting it after each call. We only need the calculated dependency degree, so we don't need to copy anything. Backpatch to PostgreSQL 10, where extended statistics were introduced. Backpatch-through: 10 Discussion: https://www.postgresql.org/message-id/20210915200928.GP831%40telsasoft.com
diff --git a/src/backend/statistics/dependencies.c b/src/backend/statistics/dependencies.c
@@ -30,6 +30,7 @@
 #include "utils/fmgroids.h"
 #include "utils/fmgrprotos.h"
 #include "utils/lsyscache.h"
+#include "utils/memutils.h"
 #include "utils/selfuncs.h"
 #include "utils/syscache.h"
 #include "utils/typcache.h"
@@ -332,13 +333,6 @@ dependency_degree(int numrows, HeapTuple *rows, int k, AttrNumber *dependency,
 		group_size++;
 	}
 
-	if (items)
-		pfree(items);
-
-	pfree(mss);
-	pfree(attnums);
-	pfree(attnums_dep);
-
 	/* Compute the 'degree of validity' as (supporting/total). */
 	return (n_supporting_rows * 1.0 / numrows);
 }
@@ -370,6 +364,7 @@ statext_dependencies_build(int numrows, HeapTuple *rows, Bitmapset *attrs,
 
 	/* result */
 	MVDependencies *dependencies = NULL;
+	MemoryContext	cxt;
 
 	/*
 	 * Transform the bms into an array, to make accessing i-th member easier.
@@ -378,6 +373,11 @@ statext_dependencies_build(int numrows, HeapTuple *rows, Bitmapset *attrs,
 
 	Assert(numattrs >= 2);
 
+	/* tracks memory allocated by dependency_degree calls */
+	cxt = AllocSetContextCreate(CurrentMemoryContext,
+								"dependency_degree cxt",
+								ALLOCSET_DEFAULT_SIZES);
+
 	/*
 	 * We'll try build functional dependencies starting from the smallest ones
 	 * covering just 2 columns, to the largest ones, covering all columns
@@ -396,10 +396,17 @@ statext_dependencies_build(int numrows, HeapTuple *rows, Bitmapset *attrs,
 		{
 			double		degree;
 			MVDependency *d;
+			MemoryContext oldcxt;
+
+			/* release memory used by dependency degree calculation */
+			oldcxt = MemoryContextSwitchTo(cxt);
 
 			/* compute how valid the dependency seems */
 			degree = dependency_degree(numrows, rows, k, dependency, stats, attrs);
 
+			MemoryContextSwitchTo(oldcxt);
+			MemoryContextReset(cxt);
+
 			/*
 			 * if the dependency seems entirely invalid, don't store it
 			 */
@@ -441,6 +448,8 @@ statext_dependencies_build(int numrows, HeapTuple *rows, Bitmapset *attrs,
 		DependencyGenerator_free(DependencyGenerator);
 	}
 
+	MemoryContextDelete(cxt);
+
 	return dependencies;
 }
 
