Fix error message when trying to alter statistics on includedcolumn

Hi,

According to the error message, it is not allowed to alter statistics on
included column because this is "non-expression column".

 postgres=# create table test (i int, d int);
 CREATE TABLE
 postgres=# create index idx on test(i) include (d);
 CREATE INDEX
 postgres=# alter index idx alter column 2 set statistics 10;
 ERROR:  cannot alter statistics on non-expression column "d" of index "idx"
 HINT:  Alter statistics on table column instead.

However, I think this should be forbidded in that this is not a key column 
but a included column. Even if we decide to support expressions in included
columns in future, it is meaningless to do this because any statistics on 
included column is never used by the planner.

Attached is the patch to fix the error message. In this fix, column number
is checked first. After applying this, the message is changed as below;

 postgres=# alter index idx alter column 2 set statistics 10;
 ERROR:  cannot alter statistics on included column "d" of index "idx"

Regards,

-- 
Yugo Nagata <nagata@sraoss.co.jp>