Generated columns

This is an SQL-standard feature that allows creating columns that are
computed from expressions rather than assigned, similar to a view or
materialized view but on a column basis.

This implements one kind of generated column: stored (computed on
write).  Another kind, virtual (computed on read), is planned for the
future, and some room is left for it.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/b151f851-4019-bdb1-699e-ebab07d2f40a@2ndquadrant.com

3 files changed, 38 insertions, 0 deletions

diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index 59e6bcd856c..10895567c09 100644
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -822,6 +822,39 @@ get_attnum(Oid relid, const char *attname)
 }
 
 /*
+ * get_attgenerated
+ *
+ *		Given the relation id and the attribute name,
+ *		return the "attgenerated" field from the attribute relation.
+ *
+ *		Errors if not found.
+ *
+ *		Since not generated is represented by '\0', this can also be used as a
+ *		Boolean test.
+ */
+char
+get_attgenerated(Oid relid, AttrNumber attnum)
+{
+	HeapTuple	tp;
+
+	tp = SearchSysCache2(ATTNUM,
+						 ObjectIdGetDatum(relid),
+						 Int16GetDatum(attnum));
+	if (HeapTupleIsValid(tp))
+	{
+		Form_pg_attribute att_tup = (Form_pg_attribute) GETSTRUCT(tp);
+		char		result;
+
+		result = att_tup->attgenerated;
+		ReleaseSysCache(tp);
+		return result;
+	}
+	else
+		elog(ERROR, "cache lookup failed for attribute %d of relation %u",
+			 attnum, relid);
+}
+
+/*
  * get_atttype
  *
  *		Given the relation OID and the attribute number with the relation,
diff --git a/src/backend/utils/cache/partcache.c b/src/backend/utils/cache/partcache.c
index 2b55f25e758..8f43d682cf3 100644
--- a/src/backend/utils/cache/partcache.c
+++ b/src/backend/utils/cache/partcache.c
@@ -27,6 +27,7 @@
 #include "nodes/nodeFuncs.h"
 #include "optimizer/optimizer.h"
 #include "partitioning/partbounds.h"
+#include "rewrite/rewriteHandler.h"
 #include "utils/builtins.h"
 #include "utils/datum.h"
 #include "utils/lsyscache.h"
diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 12f813f0bc5..64f3c2e8870 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -515,6 +515,7 @@ RelationBuildTupleDesc(Relation relation)
 	constr = (TupleConstr *) MemoryContextAlloc(CacheMemoryContext,
 												sizeof(TupleConstr));
 	constr->has_not_null = false;
+	constr->has_generated_stored = false;
 
 	/*
 	 * Form a scan key that selects only user attributes (attnum > 0).
@@ -567,6 +568,8 @@ RelationBuildTupleDesc(Relation relation)
 		/* Update constraint/default info */
 		if (attp->attnotnull)
 			constr->has_not_null = true;
+		if (attp->attgenerated == ATTRIBUTE_GENERATED_STORED)
+			constr->has_generated_stored = true;
 
 		/* If the column has a default, fill it into the attrdef array */
 		if (attp->atthasdef)
@@ -3281,6 +3284,7 @@ RelationBuildLocalRelation(const char *relname,
 		Form_pg_attribute datt = TupleDescAttr(rel->rd_att, i);
 
 		datt->attidentity = satt->attidentity;
+		datt->attgenerated = satt->attgenerated;
 		datt->attnotnull = satt->attnotnull;
 		has_not_null |= satt->attnotnull;
 	}