[CF 5700] v1 - minor error message enhancement in refuseDupeIndexAttach

This branch was automatically generated by a robot using patches from an
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https://commitfest.postgresql.org/patch/5700

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Patch(es): https://www.postgresql.org/message-id/CACJufxGBfykJ_1ztk9T+L_gLmkOSOF+mL9Mn4ZPydz-rh=LccQ@mail.gmail.com
Author(s): Jian He

Author: Commitfest Bot

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