Fix bug around assignment expressions containing indirections.

Handling of assigned-to expressions with indirection (e.g. set f1[1] =
3) was broken for ON CONFLICT DO UPDATE.  The problem was that
ParseState was consulted to determine if an INSERT-appropriate or
UPDATE-appropriate behavior should be used when transforming expressions
with indirections. When the wrong path was taken the old row was
substituted with NULL, leading to wrong results..

To fix remove p_is_update and only use p_is_insert to decide how to
transform the assignment expression, and uset p_is_insert while parsing
the on conflict statement. This isn't particularly pretty, but it's not
any worse than before.

Author: Peter Geoghegan, slightly edited by me
Discussion: CAM3SWZS8RPvA=KFxADZWw3wAHnnbxMxDzkEC6fNaFc7zSm411w@mail.gmail.com
Backpatch: 9.5, where the feature was introduced

src/test/regress/expected/arrays.out

@@ -1116,6 +1116,27 @@ select * from arr_tbl where f1 >= '{1,2,3}' and f1 < '{1,5,3}';
  {1,2,10}
 (2 rows)
 
+-- test ON CONFLICT DO UPDATE with arrays
+create temp table arr_pk_tbl (pk int4 primary key, f1 int[]);
+insert into arr_pk_tbl values (1, '{1,2,3}');
+insert into arr_pk_tbl values (1, '{3,4,5}') on conflict (pk)
+  do update set f1[1] = excluded.f1[1], f1[3] = excluded.f1[3]
+  returning pk, f1;
+ pk |   f1    
+----+---------
+  1 | {3,2,5}
+(1 row)
+
+insert into arr_pk_tbl(pk, f1[1:2]) values (1, '{6,7,8}') on conflict (pk)
+  do update set f1[1] = excluded.f1[1],
+    f1[2] = excluded.f1[2],
+    f1[3] = excluded.f1[3]
+  returning pk, f1;
+ pk |     f1     
+----+------------
+  1 | {6,7,NULL}
+(1 row)
+
 -- note: if above selects don't produce the expected tuple order,
 -- then you didn't get an indexscan plan, and something is busted.
 reset enable_seqscan;

src/test/regress/sql/arrays.sql

@@ -306,6 +306,19 @@ set enable_seqscan to off;
 set enable_bitmapscan to off;
 select * from arr_tbl where f1 > '{1,2,3}' and f1 <= '{1,5,3}';
 select * from arr_tbl where f1 >= '{1,2,3}' and f1 < '{1,5,3}';
+
+-- test ON CONFLICT DO UPDATE with arrays
+create temp table arr_pk_tbl (pk int4 primary key, f1 int[]);
+insert into arr_pk_tbl values (1, '{1,2,3}');
+insert into arr_pk_tbl values (1, '{3,4,5}') on conflict (pk)
+  do update set f1[1] = excluded.f1[1], f1[3] = excluded.f1[3]
+  returning pk, f1;
+insert into arr_pk_tbl(pk, f1[1:2]) values (1, '{6,7,8}') on conflict (pk)
+  do update set f1[1] = excluded.f1[1],
+    f1[2] = excluded.f1[2],
+    f1[3] = excluded.f1[3]
+  returning pk, f1;
+
 -- note: if above selects don't produce the expected tuple order,
 -- then you didn't get an indexscan plan, and something is busted.
 reset enable_seqscan;