Release memory allocated by dependency_degree

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

1 files changed, 17 insertions, 5 deletions

diff --git a/src/backend/statistics/dependencies.c b/src/backend/statistics/dependencies.c
index e7f2d6c0ca7..445a0db7c5e 100644
--- a/src/backend/statistics/dependencies.c
+++ b/src/backend/statistics/dependencies.c
@@ -29,6 +29,8 @@
 #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"
 
@@ -321,11 +323,6 @@ dependency_degree(int numrows, HeapTuple *rows, int k, AttrNumber *dependency,
 		group_size++;
 	}
 
-	pfree(items);
-	pfree(values);
-	pfree(isnull);
-	pfree(mss);
-
 	/* Compute the 'degree of validity' as (supporting/total). */
 	return (n_supporting_rows * 1.0 / numrows);
 }
@@ -358,6 +355,7 @@ statext_dependencies_build(int numrows, HeapTuple *rows, Bitmapset *attrs,
 
 	/* result */
 	MVDependencies *dependencies = NULL;
+	MemoryContext	cxt;
 
 	numattrs = bms_num_members(attrs);
 
@@ -372,6 +370,11 @@ 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
@@ -390,10 +393,17 @@ 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
 			 */
@@ -435,6 +445,8 @@ statext_dependencies_build(int numrows, HeapTuple *rows, Bitmapset *attrs,
 		DependencyGenerator_free(DependencyGenerator);
 	}
 
+	MemoryContextDelete(cxt);
+
 	return dependencies;
 }