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

1 files changed, 30 insertions, 1 deletions

diff --git a/src/backend/commands/copy.c b/src/backend/commands/copy.c
index 705df8900b..27d3a012af 100644
--- a/src/backend/commands/copy.c
+++ b/src/backend/commands/copy.c
@@ -32,6 +32,7 @@
 #include "commands/trigger.h"
 #include "executor/execPartition.h"
 #include "executor/executor.h"
+#include "executor/nodeModifyTable.h"
 #include "executor/tuptable.h"
 #include "foreign/fdwapi.h"
 #include "libpq/libpq.h"
@@ -2923,6 +2924,21 @@ CopyFrom(CopyState cstate)
 			else
 			{
 				/*
+				 * Compute stored generated columns
+				 *
+				 * Switch memory context so that the new tuple is in the same
+				 * context as the old one.
+				 */
+				if (resultRelInfo->ri_RelationDesc->rd_att->constr &&
+					resultRelInfo->ri_RelationDesc->rd_att->constr->has_generated_stored)
+				{
+					ExecComputeStoredGenerated(estate, slot);
+					MemoryContextSwitchTo(batchcontext);
+					tuple = ExecCopySlotHeapTuple(slot);
+					MemoryContextSwitchTo(oldcontext);
+				}
+
+				/*
 				 * If the target is a plain table, check the constraints of
 				 * the tuple.
 				 */
@@ -3271,7 +3287,7 @@ BeginCopyFrom(ParseState *pstate,
 		fmgr_info(in_func_oid, &in_functions[attnum - 1]);
 
 		/* Get default info if needed */
-		if (!list_member_int(cstate->attnumlist, attnum))
+		if (!list_member_int(cstate->attnumlist, attnum) && !att->attgenerated)
 		{
 			/* attribute is NOT to be copied from input */
 			/* use default value if one exists */
@@ -4876,6 +4892,11 @@ CopyAttributeOutCSV(CopyState cstate, char *string,
  * or NIL if there was none (in which case we want all the non-dropped
  * columns).
  *
+ * We don't include generated columns in the generated full list and we don't
+ * allow them to be specified explicitly.  They don't make sense for COPY
+ * FROM, but we could possibly allow them for COPY TO.  But this way it's at
+ * least ensured that whatever we copy out can be copied back in.
+ *
  * rel can be NULL ... it's only used for error reports.
  */
 static List *
@@ -4893,6 +4914,8 @@ CopyGetAttnums(TupleDesc tupDesc, Relation rel, List *attnamelist)
 		{
 			if (TupleDescAttr(tupDesc, i)->attisdropped)
 				continue;
+			if (TupleDescAttr(tupDesc, i)->attgenerated)
+				continue;
 			attnums = lappend_int(attnums, i + 1);
 		}
 	}
@@ -4917,6 +4940,12 @@ CopyGetAttnums(TupleDesc tupDesc, Relation rel, List *attnamelist)
 					continue;
 				if (namestrcmp(&(att->attname), name) == 0)
 				{
+					if (att->attgenerated)
+						ereport(ERROR,
+								(errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
+								 errmsg("column \"%s\" is a generated column",
+										name),
+								 errdetail("Generated columns cannot be used in COPY.")));
 					attnum = att->attnum;
 					break;
 				}