sql: INSERT ON CONFLICT DO NOTHING reports incorrect number of inserted rows

[jordanlewis]

root@:26257/test> create table test (a int primary key);
CREATE TABLE
root@:26257/test> insert into test values (1) on conflict(a) do nothing;
INSERT 1
root@:26257/test> insert into test values (1) on conflict(a) do nothing;
INSERT 1

The second insert should return INSERT 0, as no inserts were performed. This is what Postgres does as well:

jordan=# create table test (a int primary key);
CREATE TABLE
jordan=# insert into test values(1) on conflict do nothing;
INSERT 0 1
jordan=# insert into test values(1) on conflict do nothing;
INSERT 0 0

Found via #20985.

[jordanlewis]

For what it's worth, it does report the correct number of rows when executed with a RETURNING option:

root@:26257/test> insert into test values (1) on conflict(a) do nothing returning a;
+---+
| a |
+---+
+---+
(0 rows)

[knz]

I fixed that in #23373