Repair memory leakage while ANALYZE-ing complex index expressions.

The general design of memory management in Postgres is that intermediate
results computed by an expression are not freed until the end of the tuple
cycle.  For expression indexes, ANALYZE has to re-evaluate each expression
for each of its sample rows, and it wasn't bothering to free intermediate
results until the end of processing of that index.  This could lead to very
substantial leakage if the intermediate results were large, as in a recent
example from Jakub Ouhrabka.  Fix by doing ResetExprContext for each sample
row.  This necessitates adding a datumCopy step to ensure that the final
expression value isn't recycled too.  Some quick testing suggests that this
change adds at worst about 10% to the time needed to analyze a table with
an expression index; which is annoying, but seems a tolerable price to pay
to avoid unexpected out-of-memory problems.

Back-patch to all supported branches.

src/backend/commands/analyze.c

@@ -695,6 +695,12 @@ compute_index_stats(Relation onerel, double totalrows,
 		{
 			HeapTuple	heapTuple = rows[rowno];
 
+			/*
+			 * Reset the per-tuple context each time, to reclaim any cruft
+			 * left behind by evaluating the predicate or index expressions.
+			 */
+			ResetExprContext(econtext);
+
 			/* Set up for predicate or expression evaluation */
 			ExecStoreTuple(heapTuple, slot, InvalidBuffer, false);
 
@@ -719,15 +725,26 @@ compute_index_stats(Relation onerel, double totalrows,
 							   isnull);
 
 				/*
-				 * Save just the columns we care about.
+				 * Save just the columns we care about.  We copy the values
+				 * into ind_context from the estate's per-tuple context.
 				 */
 				for (i = 0; i < attr_cnt; i++)
 				{
 					VacAttrStats *stats = thisdata->vacattrstats[i];
 					int			attnum = stats->attr->attnum;
 
-					exprvals[tcnt] = values[attnum - 1];
+					if (isnull[attnum - 1])
-					exprnulls[tcnt] = isnull[attnum - 1];
+					{
+						exprvals[tcnt] = (Datum) 0;
+						exprnulls[tcnt] = true;
+					}
+					else
+					{
+						exprvals[tcnt] = datumCopy(values[attnum - 1],
+												   stats->attrtype->typbyval,
+												   stats->attrtype->typlen);
+						exprnulls[tcnt] = false;
+					}
 					tcnt++;
 				}
 			}