Re-implement pl/pgsql's expression and assignment parsing.

Invent new RawParseModes that allow the core grammar to handle
pl/pgsql expressions and assignments directly, and thereby get rid
of a lot of hackery in pl/pgsql's parser.  This moves a good deal
of knowledge about pl/pgsql into the core code: notably, we have to
invent a CoercionContext that matches pl/pgsql's (rather dubious)
historical behavior for assignment coercions.  That's getting away
from the original idea of pl/pgsql as an arm's-length extension of
the core, but really we crossed that bridge a long time ago.

The main advantage of doing this is that we can now use the core
parser to generate FieldStore and/or SubscriptingRef nodes to handle
assignments to pl/pgsql variables that are records or arrays.  That
fixes a number of cases that had never been implemented in pl/pgsql
assignment, such as nested records and array slicing, and it allows
pl/pgsql assignment to support the datatype-specific subscripting
behaviors introduced in commit c7aba7c14.

There are cosmetic benefits too: when a syntax error occurs in a
pl/pgsql expression, the error report no longer includes the confusing
"SELECT" keyword that used to get prefixed to the expression text.
Also, there seem to be some small speed gains.

Discussion: https://postgr.es/m/4165684.1607707277@sss.pgh.pa.us

1 files changed, 19 insertions, 0 deletions

diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index a0f37e52687..dc2bb40926a 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -1675,6 +1675,25 @@ typedef struct SetOperationStmt
 } SetOperationStmt;
 
 
+/* ----------------------
+ *		PL/pgSQL Assignment Statement
+ *
+ * Like SelectStmt, this is transformed into a SELECT Query.
+ * However, the targetlist of the result looks more like an UPDATE.
+ * ----------------------
+ */
+typedef struct PLAssignStmt
+{
+	NodeTag		type;
+
+	char	   *name;			/* initial column name */
+	List	   *indirection;	/* subscripts and field names, if any */
+	int			nnames;			/* number of names to use in ColumnRef */
+	SelectStmt *val;			/* the PL/pgSQL expression to assign */
+	int			location;		/* name's token location, or -1 if unknown */
+} PLAssignStmt;
+
+
 /*****************************************************************************
  *		Other Statements (no optimizations required)
  *