Allow empty target list in SELECT.

This fixes a problem noted as a followup to bug #8648: if a query has a
semantically-empty target list, e.g. SELECT * FROM zero_column_table,
ruleutils.c will dump it as a syntactically-empty target list, which was
not allowed.  There doesn't seem to be any reliable way to fix this by
hacking ruleutils (note in particular that the originally zero-column table
might since have had columns added to it); and even if we had such a fix,
it would do nothing for existing dump files that might contain bad syntax.
The best bet seems to be to relax the syntactic restriction.

Also, add parse-analysis errors for SELECT DISTINCT with no columns (after
*-expansion) and RETURNING with no columns.  These cases previously
produced unexpected behavior because the parsed Query looked like it had
no DISTINCT or RETURNING clause, respectively.  If anyone ever offers
a plausible use-case for this, we could work a bit harder on making the
situation distinguishable.

Arguably this is a bug fix that should be back-patched, but I'm worried
that there may be client apps or PLs that expect "SELECT ;" to throw a
syntax error.  The issue doesn't seem important enough to risk changing
behavior in minor releases.

2 files changed, 14 insertions, 24 deletions

diff --git a/src/test/regress/expected/errors.out b/src/test/regress/expected/errors.out
index 40615129778..5f8868da26e 100644
--- a/src/test/regress/expected/errors.out
+++ b/src/test/regress/expected/errors.out
@@ -15,26 +15,24 @@ select 1;
 --
 --
 -- SELECT
--- missing relation name
+-- this used to be a syntax error, but now we allow an empty target list
 select;
-ERROR:  syntax error at or near ";"
-LINE 1: select;
-              ^
+--
+(1 row)
+
 -- no such relation
 select * from nonesuch;
 ERROR:  relation "nonesuch" does not exist
 LINE 1: select * from nonesuch;
                       ^
--- missing target list
-select from pg_database;
-ERROR:  syntax error at or near "from"
-LINE 1: select from pg_database;
-               ^
 -- bad name in target list
 select nonesuch from pg_database;
 ERROR:  column "nonesuch" does not exist
 LINE 1: select nonesuch from pg_database;
                ^
+-- empty distinct list isn't OK
+select distinct from pg_database;
+ERROR:  SELECT DISTINCT must have at least one column
 -- bad attribute name on lhs of operator
 select * from pg_database where nonesuch = pg_database.datname;
 ERROR:  column "nonesuch" does not exist
@@ -45,12 +43,7 @@ select * from pg_database where pg_database.datname = nonesuch;
 ERROR:  column "nonesuch" does not exist
 LINE 1: ...ect * from pg_database where pg_database.datname = nonesuch;
                                                               ^
--- bad select distinct on syntax, distinct attribute missing
-select distinct on (foobar) from pg_database;
-ERROR:  syntax error at or near "from"
-LINE 1: select distinct on (foobar) from pg_database;
-                                    ^
--- bad select distinct on syntax, distinct attribute not in target list
+-- bad attribute name in select distinct on
 select distinct on (foobar) * from pg_database;
 ERROR:  column "foobar" does not exist
 LINE 1: select distinct on (foobar) * from pg_database;
diff --git a/src/test/regress/sql/errors.sql b/src/test/regress/sql/errors.sql
index 2ee707c5c75..cd370b4781e 100644
--- a/src/test/regress/sql/errors.sql
+++ b/src/test/regress/sql/errors.sql
@@ -16,28 +16,25 @@ select 1;
 --
 -- SELECT
 
--- missing relation name
+-- this used to be a syntax error, but now we allow an empty target list
 select;
 
 -- no such relation
 select * from nonesuch;
 
--- missing target list
-select from pg_database;
 -- bad name in target list
 select nonesuch from pg_database;
+
+-- empty distinct list isn't OK
+select distinct from pg_database;
+
 -- bad attribute name on lhs of operator
 select * from pg_database where nonesuch = pg_database.datname;
 
 -- bad attribute name on rhs of operator
 select * from pg_database where pg_database.datname = nonesuch;
 
-
--- bad select distinct on syntax, distinct attribute missing
-select distinct on (foobar) from pg_database;
-
-
--- bad select distinct on syntax, distinct attribute not in target list
+-- bad attribute name in select distinct on
 select distinct on (foobar) * from pg_database;