psql: Tab completion for JOIN ... USING column list

For JOIN ... USING, offer attribute names for the first member of the
column list.

Author: Andreas Karlsson
Reviewed-By: Tomas Vondra
Discussion: https://postgr.es/m/3a7e27bc-d6ed-4cb0-9b21-f21143fc1b37@proxel.se

src/bin/psql/tab-complete.in.c

@@ -5179,6 +5179,10 @@ match_previous_words(int pattern_id,
 			 TailMatches("JOIN", MatchAny, MatchAny, "USING") ||
 			 TailMatches("JOIN", "LATERAL", MatchAny, MatchAny, "USING"))
 		COMPLETE_WITH("(");
+	else if (TailMatches("JOIN", MatchAny, "USING", "("))
+		COMPLETE_WITH_ATTR(prev3_wd);
+	else if (TailMatches("JOIN", MatchAny, MatchAny, "USING", "("))
+		COMPLETE_WITH_ATTR(prev4_wd);
 
 /* ... AT [ LOCAL | TIME ZONE ] ... */
 	else if (TailMatches("AT"))