Change unknown-type literals to type text in SELECT and RETURNING lists.

Previously, we left such literals alone if the query or subquery had
no properties forcing a type decision to be made (such as an ORDER BY or
DISTINCT clause using that output column).  This meant that "unknown" could
be an exposed output column type, which has never been a great idea because
it could result in strange failures later on.  For example, an outer query
that tried to do any operations on an unknown-type subquery output would
generally fail with some weird error like "failed to find conversion
function from unknown to text" or "could not determine which collation to
use for string comparison".  Also, if the case occurred in a CREATE VIEW's
query then the view would have an unknown-type column, causing similar
failures in queries trying to use the view.

To fix, at the tail end of parse analysis of a query, forcibly convert any
remaining "unknown" literals in its SELECT or RETURNING list to type text.
However, provide a switch to suppress that, and use it in the cases of
SELECT inside a set operation or INSERT command.  In those cases we already
had type resolution rules that make use of context information from outside
the subquery proper, and we don't want to change that behavior.

Also, change creation of an unknown-type column in a relation from a
warning to a hard error.  The error should be unreachable now in CREATE
VIEW or CREATE MATVIEW, but it's still possible to explicitly say "unknown"
in CREATE TABLE or CREATE (composite) TYPE.  We want to forbid that because
it's nothing but a foot-gun.

This change creates a pg_upgrade failure case: a matview that contains an
unknown-type column can't be pg_upgraded, because reparsing the matview's
defining query will now decide that the column is of type text, which
doesn't match the cstring-like storage that the old materialized column
would actually have.  Add a checking pass to detect that.  While at it,
we can detect tables or composite types that would fail, essentially
for free.  Those would fail safely anyway later on, but we might as
well fail earlier.

This patch is by me, but it owes something to previous investigations
by Rahila Syed.  Also thanks to Ashutosh Bapat and Michael Paquier for
review.

Discussion: https://postgr.es/m/CAH2L28uwwbL9HUM-WR=hromW1Cvamkn7O-g8fPY2m=_7muJ0oA@mail.gmail.com

3 files changed, 8 insertions, 1 deletions

diff --git a/src/include/parser/analyze.h b/src/include/parser/analyze.h
index a7e5c55ab4f..17259409a72 100644
--- a/src/include/parser/analyze.h
+++ b/src/include/parser/analyze.h
@@ -29,7 +29,8 @@ extern Query *parse_analyze_varparams(RawStmt *parseTree, const char *sourceText
 
 extern Query *parse_sub_analyze(Node *parseTree, ParseState *parentParseState,
 				  CommonTableExpr *parentCTE,
-				  bool locked_from_parent);
+				  bool locked_from_parent,
+				  bool resolve_unknowns);
 
 extern Query *transformTopLevelStmt(ParseState *pstate, RawStmt *parseTree);
 extern Query *transformStmt(ParseState *pstate, Node *parseTree);
diff --git a/src/include/parser/parse_node.h b/src/include/parser/parse_node.h
index bc3eea9ba5a..3a25d9598d6 100644
--- a/src/include/parser/parse_node.h
+++ b/src/include/parser/parse_node.h
@@ -150,6 +150,9 @@ typedef Node *(*CoerceParamHook) (ParseState *pstate, Param *param,
  * p_locked_from_parent: true if parent query level applies FOR UPDATE/SHARE
  * to this subquery as a whole.
  *
+ * p_resolve_unknowns: resolve unknown-type SELECT output columns as type TEXT
+ * (this is true by default).
+ *
  * p_hasAggs, p_hasWindowFuncs, etc: true if we've found any of the indicated
  * constructs in the query.
  *
@@ -182,6 +185,8 @@ struct ParseState
 	List	   *p_locking_clause;		/* raw FOR UPDATE/FOR SHARE info */
 	bool		p_locked_from_parent;	/* parent has marked this subquery
 										 * with FOR UPDATE/FOR SHARE */
+	bool		p_resolve_unknowns;		/* resolve unknown-type SELECT outputs
+										 * as type text */
 
 	/* Flags telling about things found in the query: */
 	bool		p_hasAggs;
diff --git a/src/include/parser/parse_target.h b/src/include/parser/parse_target.h
index e035aacbf4f..d06a235df0b 100644
--- a/src/include/parser/parse_target.h
+++ b/src/include/parser/parse_target.h
@@ -21,6 +21,7 @@ extern List *transformTargetList(ParseState *pstate, List *targetlist,
 					ParseExprKind exprKind);
 extern List *transformExpressionList(ParseState *pstate, List *exprlist,
 						ParseExprKind exprKind, bool allowDefault);
+extern void resolveTargetListUnknowns(ParseState *pstate, List *targetlist);
 extern void markTargetListOrigins(ParseState *pstate, List *targetlist);
 extern TargetEntry *transformTargetEntry(ParseState *pstate,
 					 Node *node, Node *expr, ParseExprKind exprKind,