Allow ALTER TYPE to change some properties of a base type.

Specifically, this patch allows ALTER TYPE to:
* Change the default TOAST strategy for a toastable base type;
* Promote a non-toastable type to toastable;
* Add/remove binary I/O functions for a type;
* Add/remove typmod I/O functions for a type;
* Add/remove a custom ANALYZE statistics functions for a type.

The first of these can be done by the type's owner; all the others
require superuser privilege since misuse could cause problems.

The main motivation for this patch is to allow extensions to
upgrade the feature sets of their data types, so the set of
alterable properties is biased towards that use-case.  However
it's also true that changing some other properties would be
a lot harder, as they get baked into physical storage and/or
stored expressions that depend on the type.

Along the way, refactor GenerateTypeDependencies() to make it easier
to call, refactor DefineType's volatility checks so they can be shared
by AlterType, and teach typcache.c that it might have to reload data
from the type's pg_type row, a scenario it never handled before.
Also rearrange alter_type.sgml a bit for clarity (put the
composite-type operations together).

Tomas Vondra and Tom Lane

Discussion: https://postgr.es/m/20200228004440.b23ein4qvmxnlpht@development

2 files changed, 12 insertions, 2 deletions

diff --git a/src/include/nodes/nodes.h b/src/include/nodes/nodes.h
index baced7eec0f..8a76afe8ccb 100644
--- a/src/include/nodes/nodes.h
+++ b/src/include/nodes/nodes.h
@@ -380,6 +380,7 @@ typedef enum NodeTag
 	T_AlterObjectSchemaStmt,
 	T_AlterOwnerStmt,
 	T_AlterOperatorStmt,
+	T_AlterTypeStmt,
 	T_DropOwnedStmt,
 	T_ReassignOwnedStmt,
 	T_CompositeTypeStmt,
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index da0706add59..2039b424499 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -2959,9 +2959,8 @@ typedef struct AlterOwnerStmt
 	RoleSpec   *newowner;		/* the new owner */
 } AlterOwnerStmt;
 
-
 /* ----------------------
- *		Alter Operator Set Restrict, Join
+ *		Alter Operator Set ( this-n-that )
  * ----------------------
  */
 typedef struct AlterOperatorStmt
@@ -2971,6 +2970,16 @@ typedef struct AlterOperatorStmt
 	List	   *options;		/* List of DefElem nodes */
 } AlterOperatorStmt;
 
+/* ------------------------
+ *		Alter Type Set ( this-n-that )
+ * ------------------------
+ */
+typedef struct AlterTypeStmt
+{
+	NodeTag		type;
+	List	   *typeName;		/* type name (possibly qualified) */
+	List	   *options;		/* List of DefElem nodes */
+} AlterTypeStmt;
 
 /* ----------------------
  *		Create Rule Statement