Fix interaction of TOAST compression with expression indexes.

Before, trying to compress a value for insertion into an expression
index would crash.

Dilip Kumar, with some editing by me. Report by Jaime Casanova.

Discussion: http://postgr.es/m/CAJKUy5gcs0zGOp6JXU2mMVdthYhuQpFk=S3V8DOKT=LZC1L36Q@mail.gmail.com

src/backend/catalog/index.c

@@ -30,6 +30,7 @@
 #include "access/relscan.h"
 #include "access/sysattr.h"
 #include "access/tableam.h"
+#include "access/toast_compression.h"
 #include "access/transam.h"
 #include "access/visibilitymap.h"
 #include "access/xact.h"
@@ -379,6 +380,15 @@ ConstructTupleDescriptor(Relation heapRelation,
 			to->attalign = typeTup->typalign;
 			to->atttypmod = exprTypmod(indexkey);
 
+			/*
+			 * For expression columns, set attcompression invalid, since
+			 * there's no table column from which to copy the value. Whenever
+			 * we actually need to compress a value, we'll use whatever the
+			 * current value of default_compression_method is at that point
+			 * in time.
+			 */
+			to->attcompression = InvalidCompressionMethod;
+
 			ReleaseSysCache(tuple);
 
 			/*