[YSQL] Fix single-row-txn detection after primary key change

Summary:
D6326 (yugabyte#955) implemented handling primary key as an index so that in can be used in an
index scan path.
As a side-effect it caused single-row inserts to be mis-categorized as multi-row operations
causing them to use the distributed txns path (instead of the single row txn path).
This fixes the relevant check.

Test Plan: java -jar ./target/yb-sample-apps.jar --workload SqlInserts --nodes 127.0.0.1:5433 --num_threads_write 24 --num_threads_read 0 --num_unique_keys 1000000000

Reviewers: robert

Reviewed By: robert

Subscribers: yql

Differential Revision: https://phabricator.dev.yugabyte.com/D6437

Author: m-iancu

src/postgres/src/backend/executor/nodeModifyTable.c

@@ -430,8 +430,13 @@ ExecInsert(ModifyTableState *mtstate,
 			bool has_triggers = resultRelInfo->ri_TrigDesc &&
 			                    resultRelInfo->ri_TrigDesc->numtriggers > 0;
 
+			bool has_indices = resultRelInfo->ri_NumIndices > 1 ||
+			                   (resultRelInfo->ri_NumIndices == 1 &&
+			                    !resultRelInfo->ri_IndexRelationDescs[0]->rd_index
+			                                                            ->indisprimary);
+
 			bool is_single_row_txn = estate->es_yb_is_single_row_modify_txn &&
-			                         resultRelInfo->ri_NumIndices == 0 &&
+			                         !has_indices &&
 			                         !has_triggers;
 
 			if (is_single_row_txn)