Fix ExecEvalArrayRef to pass down the old value of the array element or slice

being assigned to, in case the expression to be assigned is a FieldStore that
would need to modify that value.  The need for this was foreseen some time
ago, but not implemented then because we did not have arrays of composites.
Now we do, but the point evidently got overlooked in that patch.  Net result
is that updating a field of an array element doesn't work right, as
illustrated if you try the new regression test on an unpatched backend.
Noted while experimenting with EXPLAIN VERBOSE, which has also got some issues
in this area.

Backpatch to 8.3, where arrays of composites were introduced.

2 files changed, 24 insertions, 0 deletions

diff --git a/src/test/regress/expected/arrays.out b/src/test/regress/expected/arrays.out
index 7225d6e4429..3ab18be9a79 100644
--- a/src/test/regress/expected/arrays.out
+++ b/src/test/regress/expected/arrays.out
@@ -1192,3 +1192,19 @@ select unnest(array[1,2,3,null,4,null,null,5,6]::text[]);
  6
 (9 rows)
 
+-- Insert/update on a column that is array of composite
+create temp table t1 (f1 int8_tbl[]);
+insert into t1 (f1[5].q1) values(42);
+select * from t1;
+       f1        
+-----------------
+ [5:5]={"(42,)"}
+(1 row)
+
+update t1 set f1[5].q2 = 43;
+select * from t1;
+        f1         
+-------------------
+ [5:5]={"(42,43)"}
+(1 row)
+
diff --git a/src/test/regress/sql/arrays.sql b/src/test/regress/sql/arrays.sql
index 5e4933c31d3..f65bc452e2e 100644
--- a/src/test/regress/sql/arrays.sql
+++ b/src/test/regress/sql/arrays.sql
@@ -404,3 +404,11 @@ select unnest(array[1,2,3,4.5]::float8[]);
 select unnest(array[1,2,3,4.5]::numeric[]);
 select unnest(array[1,2,3,null,4,null,null,5,6]);
 select unnest(array[1,2,3,null,4,null,null,5,6]::text[]);
+
+-- Insert/update on a column that is array of composite
+
+create temp table t1 (f1 int8_tbl[]);
+insert into t1 (f1[5].q1) values(42);
+select * from t1;
+update t1 set f1[5].q2 = 43;
+select * from t1;