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From "Daniel John Debrunner (JIRA)" <derby-...@db.apache.org>
Subject [jira] Commented: (DERBY-438) Update triggers on tables with BLOB and CLOB columns fail at execution time if the triggered-SQL-statement references the LOB column(s).
Date Tue, 02 May 2006 22:41:50 GMT
    [ http://issues.apache.org/jira/browse/DERBY-438?page=comments#action_12377483 ] 

Daniel John Debrunner commented on DERBY-438:
---------------------------------------------

Several fixes have been made (seen the Subversion commits) that support blob/clob in row triggers
including the case when the LOB value is streamed from the store.
Writing a test for statement triggers showed an issue where the implementation of the statement
trigger depended on a VTI (ResultSet implementation) that was still at JDBC 1.2 level, thus
not supporting BLOB/CLOB. Some description of how triggers are executed has been documented
on the wiki at:

http://wiki.apache.org/db-derby/TriggerImplementation

The proposed change to fix this is to change the VTIs used in triggers from implementations
of java.sql.ResultSet that wrap a embedded java.sql.ResultSet  that wraps a Derby language
ResultSet, to implementations of java.sql.PreparedStatement  that simply return the embedded
java.sql.ResultSet  that wraps a Derby language ResultSet. 

PreparedStatement VTIs are more flexible that ResultSet  VTIs in that they can be updatable
(not required in this case) and (for this case) the implementation of the ResultSet can be
hidden by the class, allowing different implementations in different situations.

This fixes the situation for statement triggers as the ResultSet now returned to the re-written
 trigger action statement  will support BLOB/CLOB as it will be JDBC ResultSet as returned
from the embedded driver. Since the api of the VTI does not change this fix should work on
soft upgrade for old triggers from 10.1/10.0 and triggers created in 10.2 soft upgrade that
will then run in 10.1/10.0

> Update triggers on tables with BLOB and CLOB columns fail at execution time if the triggered-SQL-statement
references the LOB column(s).
> ----------------------------------------------------------------------------------------------------------------------------------------
>
>          Key: DERBY-438
>          URL: http://issues.apache.org/jira/browse/DERBY-438
>      Project: Derby
>         Type: Bug

>   Components: SQL
>     Versions: 10.1.1.0, 10.2.0.0, 10.0.2.2
>     Reporter: A B
>     Assignee: Daniel John Debrunner
>      Fix For: 10.2.0.0
>  Attachments: go.java
>
> Suppose I have 1) a table "t1" with blob data in it, and 2) an UPDATE trigger "tr1" defined
on that table, where the triggered-SQL-action for "tr1" references the blob column from the
updated ("new") row. Ex:
> create table t1 (id int, updated smallint, bl blob(32000));
> create trigger tr1 after update on t1 referencing new as n_row for each row mode db2sql
values length(n_row.bl);
> Assuming that t1 has been populated with some data, then attempts to update t1 will fire
the trigger, but the result will be one of the two following errors:
> 1) If blob data is < 32K...
> If the actual data in the table is less than 32K in length, the result will be:
> ERROR XCL12: An attempt was made to put a data value of type 'org.apache.derby.impl.jdbc.EmbedBlob'
into a data value of type 'BLOB'.
> 2) If blob data is > 32K...
> If at least one row in the table has blob data that is longer than 32K (which means that
Derby will stream it, so far as I can tell), then the error will be:
> ERROR XCL30: An IOException was thrown when reading a 'BLOB' from an InputStream.
> ERROR XJ001: Java exception: ': java.io.EOFException'.
> Note that for data larger than 32K, this error will occur regardless of whether or not
the triggered-SQL-statement
> references the blob column.
> Surprisingly, it doesn't (appear to) matter what the trigger statement is actually doing--so
long as it references the blob column at least once, one of these two errors will occur, depending
on the length of the data.  And if the data is greater than 32k, then the error will happen
regardless of what the trigger does or whether or not it references the blob column.
> I looked at the documentation for UPDATE statements and TRIGGER statements, but nowhere
did I see anything saying that either of these will not work with blobs.  So as far as I can
tell, both of the above scenarios should succeed...

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