minor error message enhancement

in refuseDupeIndexAttach change it
from
errdetail("Another index is already attached for partition \"%s\"."...)
to
errdetail("Another index \"%s\" is already attached for partition \"%s\".")

So we can easily understand which index is already attached for partition \"%s\".

discussion: https://postgr.es/m/

Author: jianhe-fun

src/backend/commands/tablecmds.c

@@ -21702,7 +21702,8 @@ refuseDupeIndexAttach(Relation parentIdx, Relation partIdx, Relation partitionTb
 				 errmsg("cannot attach index \"%s\" as a partition of index \"%s\"",
 						RelationGetRelationName(partIdx),
 						RelationGetRelationName(parentIdx)),
-				 errdetail("Another index is already attached for partition \"%s\".",
+				 errdetail("Another index \"%s\" is already attached for partition \"%s\".",
+						   get_rel_name(existingIdx),
 						   RelationGetRelationName(partitionTbl))));
 }
 

src/test/regress/expected/indexing.out

@@ -248,7 +248,7 @@ alter index idxpart_a_b_idx attach partition idxpart1_a_b_idx; -- quiet
 create index idxpart1_2_a_b on idxpart1 (a, b);
 alter index idxpart_a_b_idx attach partition idxpart1_2_a_b;
 ERROR:  cannot attach index "idxpart1_2_a_b" as a partition of index "idxpart_a_b_idx"
-DETAIL:  Another index is already attached for partition "idxpart1".
+DETAIL:  Another index "idxpart1_a_b_idx" is already attached for partition "idxpart1".
 drop table idxpart;
 -- make sure everything's gone
 select indexrelid::regclass, indrelid::regclass