Modify the infer_arbiter_indexes function to consider both indisvalid and indisready indexes. Ensure that at least one indisvalid index is still required.

The change ensures that all concurrent transactions utilize the same set of indexes as arbiters. This uniformity is required to avoid conditions that could lead to "duplicate key value violates unique constraint" errors during UPSERT operations.

The patch resolves the issues in the following specs:
* reindex_concurrently_upsert
* index_concurrently_upsert
* index_concurrently_upsert_predicate

Despite the patch, the following specs are still affected:
* reindex_concurrently_upsert_partitioned
* reindex_concurrently_upsert_on_constraint

Author: michail-nikolaev

src/backend/optimizer/util/plancat.c

@@ -814,6 +814,7 @@ infer_arbiter_indexes(PlannerInfo *root)
 
 	/* Results */
 	List	   *results = NIL;
+	bool	   foundValid = false;
 
 	/*
 	 * Quickly return NIL for ON CONFLICT DO NOTHING without an inference
@@ -907,7 +908,13 @@ infer_arbiter_indexes(PlannerInfo *root)
 		idxRel = index_open(indexoid, rte->rellockmode);
 		idxForm = idxRel->rd_index;
 
-		if (!idxForm->indisvalid)
+		/*
+		 * We need to consider both indisvalid and indisready indexes because
+		 * them may become indisvalid before execution phase. It is required
+		 * to keep set of indexes used as arbiter to be the same for all
+		 * concurrent transactions.
+		 */
+		if (!idxForm->indisready)
 			goto next;
 
 		/*
@@ -929,10 +936,9 @@ infer_arbiter_indexes(PlannerInfo *root)
 						 errmsg("ON CONFLICT DO UPDATE not supported with exclusion constraints")));
 
 			results = lappend_oid(results, idxForm->indexrelid);
-			list_free(indexList);
+			foundValid |= idxForm->indisvalid;
 			index_close(idxRel, NoLock);
-			table_close(relation, NoLock);
-			return results;
+			break;
 		}
 		else if (indexOidFromConstraint != InvalidOid)
 		{
@@ -1033,14 +1039,16 @@ infer_arbiter_indexes(PlannerInfo *root)
 			goto next;
 
 		results = lappend_oid(results, idxForm->indexrelid);
+		foundValid |= idxForm->indisvalid;
 next:
 		index_close(idxRel, NoLock);
 	}
 
 	list_free(indexList);
 	table_close(relation, NoLock);
 
-	if (results == NIL)
+	/* It is required to have at least one indisvalid index during the planning. */
+	if (results == NIL || !foundValid)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
 				 errmsg("there is no unique or exclusion constraint matching the ON CONFLICT specification")));