Add support for coordinating record typmods among parallel workers.

Tuples can have type RECORDOID and a typmod number that identifies a blessed
TupleDesc in a backend-private cache.  To support the sharing of such tuples
through shared memory and temporary files, provide a typmod registry in
shared memory.

To achieve that, introduce per-session DSM segments, created on demand when a
backend first runs a parallel query.  The per-session DSM segment has a
table-of-contents just like the per-query DSM segment, and initially the
contents are a shared record typmod registry and a DSA area to provide the
space it needs to grow.

State relating to the current session is accessed via a Session object
reached through global variable CurrentSession that may require significant
redesign further down the road as we figure out what else needs to be shared
or remodelled.

Author: Thomas Munro
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAEepm=0ZtQ-SpsgCyzzYpsXS6e=kZWqk3g5Ygn3MDV7A8dabUA@mail.gmail.com

1 files changed, 16 insertions, 0 deletions

diff --git a/src/backend/access/common/tupdesc.c b/src/backend/access/common/tupdesc.c
index 4436c863617..9e37ca73a86 100644
--- a/src/backend/access/common/tupdesc.c
+++ b/src/backend/access/common/tupdesc.c
@@ -185,6 +185,22 @@ CreateTupleDescCopyConstr(TupleDesc tupdesc)
 }
 
 /*
+ * TupleDescCopy
+ *		Copy a tuple descriptor into caller-supplied memory.
+ *		The memory may be shared memory mapped at any address, and must
+ *		be sufficient to hold TupleDescSize(src) bytes.
+ *
+ * !!! Constraints and defaults are not copied !!!
+ */
+void
+TupleDescCopy(TupleDesc dst, TupleDesc src)
+{
+	memcpy(dst, src, TupleDescSize(src));
+	dst->constr = NULL;
+	dst->tdrefcount = -1;
+}
+
+/*
  * TupleDescCopyEntry
  *		This function copies a single attribute structure from one tuple
  *		descriptor to another.