Clarify comment about assignment and reset of temp namespace ID in MyProc

The new wording comes from Álvaro, which I modified a bit.

Reported-by: Andres Freund, Álvaro Herrera
Author: Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/20180809165047.GK13638@paquier.xyz
Backpatch-through: 11

diff --git a/src/backend/catalog/namespace.c b/src/backend/catalog/namespace.c
index 3971346e738d1d8e0c0fce89587a4cb51abb598b..5d13e6a3d70142016f5219c86fc5f4fc1c67c8f6 100644 (file)
--- a/src/backend/catalog/namespace.c
+++ b/src/backend/catalog/namespace.c
@@ -3938,8 +3938,10 @@ InitTempTableNamespace(void)
     * decide if a temporary namespace is in use or not.  We assume that
     * assignment of namespaceId is an atomic operation.  Even if it is not,
     * the temporary relation which resulted in the creation of this temporary
-    * namespace is still locked until the current transaction commits, so it
-    * would not be accessible yet, acting as a barrier.
+    * namespace is still locked until the current transaction commits, and
+    * its pg_namespace row is not visible yet.  However it does not matter:
+    * this flag makes the namespace as being in use, so no objects created on
+    * it would be removed concurrently.
     */
    MyProc->tempNamespaceId = namespaceId;
 
@@ -3976,10 +3978,12 @@ AtEOXact_Namespace(bool isCommit, bool parallel)
 
            /*
             * Reset the temporary namespace flag in MyProc.  We assume that
-            * this operation is atomic.  Even if it is not, the temporary
-            * table which created this namespace is still locked until this
-            * transaction aborts so it would not be visible yet, acting as a
-            * barrier.
+            * this operation is atomic.
+            *
+            * Because this transaction is aborting, the pg_namespace row is
+            * not visible to anyone else anyway, but that doesn't matter:
+            * it's not a problem if objects contained in this namespace are
+            * removed concurrently.
             */
            MyProc->tempNamespaceId = InvalidOid;
        }
@@ -4037,10 +4041,12 @@ AtEOSubXact_Namespace(bool isCommit, SubTransactionId mySubid,
 
            /*
             * Reset the temporary namespace flag in MyProc.  We assume that
-            * this operation is atomic.  Even if it is not, the temporary
-            * table which created this namespace is still locked until this
-            * transaction aborts so it would not be visible yet, acting as a
-            * barrier.
+            * this operation is atomic.
+            *
+            * Because this subtransaction is aborting, the pg_namespace row
+            * is not visible to anyone else anyway, but that doesn't matter:
+            * it's not a problem if objects contained in this namespace are
+            * removed concurrently.
             */
            MyProc->tempNamespaceId = InvalidOid;
        }