Why do I get "ORA-00932: inconsistent datatypes: expected - got -" when using COLLECT() in a prepared statement?

Question

I am using this query with the Perl DBI:

SELECT c.change_id
     , COLLECT(t.tag) AS the_tags
  FROM changes   c
  LEFT JOIN tags t ON c.change_id = t.change_id
 WHERE c.project = ?
 GROUP BY c.change_id

The DBI uses OCI to prepare this statement, bind the value I pass, and get the results. But Oracle, for some reason, does not like it. The error output is:

     ORA-00932: inconsistent datatypes: expected - got - (DBD ERROR: error possibly near <*> indicator at char 41 in '
            SELECT c.change_id
                 , <*>COLLECT(t.tag) AS the_tags
              FROM changes   c
              LEFT JOIN tags t ON c.change_id = t.change_id
             WHERE c.project = :p1
             GROUP BY c.change_id
        '

Not very informative. However, I can make this error go away not only by changing the call to COLLECT() also by replacing the placeholder with the actual value:

     SELECT c.change_id
          , COLLECT(t.tag) AS the_tags
       FROM changes   c
       LEFT JOIN tags t ON c.change_id = t.change_id
      WHERE c.project = 'tryoracle'
      GROUP BY c.change_id

That version works perfectly. Why doesn't Oracle like the prepared statement with the COLLECT()?

In case it's any help, here is a trace of the OCI-related calls extracted via ora_verbose = 6 (h/t @bohica).

Solution

Finally got a solution to this issue, thanks to some digging by a user. The problem was not with the placeholder; why it worked without the placeholder on the VirtualBox image I have no idea. No, the issue was with the COLLECT(). Seems that both the values being collected need to be cast to a specific type, and the resulting array also needs to be cast to a pre-defined array data type. Just so happens that my code has a custom array type:

CREATE TYPE sqitch_array AS varray(1024) OF VARCHAR2(512);

So I'm able to get the query to work by casting the COLLECT() like so:

CAST(COLLECT(CAST(t.tags as VARCHAR2(512))) AS sqitch_array)

Comments

[Kurt Mueller]

Is there a way to do this in one line? Something along the lines of CAST(COLLECT(CAST(t.tags as VARCHAR2(512))) AS (varray(1024) OF VARCHAR2(512)))?

[Kjetil S.]

I mysteriously got ORA-00932 with select distinct myid, myblobcolumn from mytable. Removing distinct helped.