From 1b4f7f93b4693858cb983af3cd557f6097dab67b Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Sat, 14 Dec 2013 20:23:26 -0500 Subject: 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. --- src/backend/parser/parse_clause.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) (limited to 'src/backend/parser/parse_clause.c') diff --git a/src/backend/parser/parse_clause.c b/src/backend/parser/parse_clause.c index 939fa834e0a..87b0c8fd418 100644 --- a/src/backend/parser/parse_clause.c +++ b/src/backend/parser/parse_clause.c @@ -2011,6 +2011,20 @@ transformDistinctClause(ParseState *pstate, true); } + /* + * Complain if we found nothing to make DISTINCT. Returning an empty list + * would cause the parsed Query to look like it didn't have DISTINCT, with + * results that would probably surprise the user. Note: this case is + * presently impossible for aggregates because of grammar restrictions, + * but we check anyway. + */ + if (result == NIL) + ereport(ERROR, + (errcode(ERRCODE_SYNTAX_ERROR), + is_agg ? + errmsg("an aggregate with DISTINCT must have at least one argument") : + errmsg("SELECT DISTINCT must have at least one column"))); + return result; } @@ -2115,6 +2129,11 @@ transformDistinctOnClause(ParseState *pstate, List *distinctlist, true); } + /* + * An empty result list is impossible here because of grammar restrictions. + */ + Assert(result != NIL); + return result; } -- cgit v1.2.3