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http://issues.apache.org/jira/browse/DERBY-208?page=comments#action_12452184 ]
Fernanda Pizzorno commented on DERBY-208:
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I am investigating possible implementations LOBs using locators instead of
materializing the LOB, and I would like to suggest the following approach.
1. MOTIVATION
In the current implementation of LOBs in the Derby Client Driver, the LOBs are
materialized on the client size. In cases where large LOBS are being accesses,
this materialization of the LOBs in the client side can cause an
OutOfMemoryError.
2. APPROACH
The network client will request LOB columns to be sent as LOB Locators instead
of data value bytes. Since the client no longer has a copy of the LOB data
value bytes, operations on the LOB data will be performed on the server side,
and the results of these operations will be sent to the client. The client will
request the execution of an operation by the server by calling a stored
procedure.
2.1 DRDA
The Output Override Descriptor (OUTOVR) allows the client to specify the output
format for data to be returned as output to an SQL statement or query. This
command data object must be sent with the command that returns the LOB data
column as output (either a CNTQRY - Continue Query, or an EXCSQLSTT Execute SQL
Statement commands). LOB columns are returned by default as data value bytes,
in order to receive the LOB columns as LOB Locators, the network client must
explicitly request it using the OUTOVR command data object.
2.2 JDBC Driver
The implementation of LOB related JDBC methods in the network client must be
converted to use LOB Locators instead of having a copy of the LOB data in the
client side. Since the client will no longer have the data value bytes for the
LOB, operations performed on the data must be performed on the server side.
This will be done using stored procedures. Each time a LOB related JDBC method
is called, the client driver will "convert" that call into a call to one or
more stored procedures. These are the necessary stored procedure to implement
LOB related JDBC methods:
* CreateLocator(LocatorType IN SMALLINT, Locator OUT INTEGER)
-> Allocates a new LOB in a temporary space and returns its Locator
* ReleaseLocator(LocatorType IN SMALLINT, Locator IN INTEGER)
-> Releases a LOB.
* GetPosition(LocatorType IN SMALLINT, Locator IN INTEGER,
SearchLocator IN INTEGER, SearchLiteral IN STRING,
FromPosition IN INTEGER, LocatedAt OUT INTEGER)
-> Returns the position in bytes/characters of the first occurrence of
SearchLocator or SearchLiteral within the LOB.
* GetLength(LocatorType IN SMALLINT, SourceLocator IN INTEGER,
Length OUT INTEGER)
-> Returns the length in bytes/characters of the LOB
* GetSubString(LocatorType IN SMALLINT, SourceLocator IN INTEGER,
FromPosition IN INTEGER, ForLength IN INTEGER,
String OUT STRING)
-> Returns a byte array or string containing the bytes/characters
starting at FromPosition and with length ForLength
* SetString(LocatorType IN SMALLINT, SourceLocator IN INTEGER,
FromPosition IN INTEGER, ForLength IN INTEGER,
String IN STRING)
-> Replaces the bytes/characters at position FromPosition with
ForLength bytes/characters from String
* Truncate(LocatorType IN SMALLINT, SourceLocator IN INTEGER,
Length IN INTEGER)
-> Truncates the LOB to Length
3. COMPATIBILITY
3.1 Old server with new client
The new network client will always request to receive the LOB columns as LOB
Locator values instead of the data values bytes. This request is done by
sending an OUTOVR object describing the desired type with the CNTQRY command.
The current implementation of the network server is able to parse these OUTOVR
objects so receiving such an object from the client will not cause an error to
happen. However, the network server does not do anything with the information
received in the OUTOVR object, so it will continue to send the LOB columns as
data value bytes instead of sending the LOB Locator.
In order to preserve compatibility between the current network server and a new
network client, the new network client must be able to handle LOB data value
bytes.
3.2 Old client with new server
The new server will be able to send LOB columns as LOB Locators instead of
sending them as LOB data values bytes, if the network client requests it. The
behavior of the old client is unchanged, so it will continue to request for the
LOB columns to be sent as LOB data value bytes.
In order to preserve compatibility between the current network client and a new
network server, the new network server must be able to return LOB columns both
as LOB Locators and as LOB data values bytes. If the client request the LOB
columns to be sent as LOB Locators, the server will return them as LOB
Locators, otherwise it will return them as LOB data value bytes. This should
also preserve the compatibility with the JCC client.
> Add support to retrieve lobs for Network Server by locator rather than
> matierializing the LOB
> ---------------------------------------------------------------------------------------------
>
> Key: DERBY-208
> URL: http://issues.apache.org/jira/browse/DERBY-208
> Project: Derby
> Issue Type: New Feature
> Components: Network Server
> Affects Versions: 10.1.1.0
> Reporter: Kathey Marsden
>
> Currently Network Server materializes all LOB objects. LOB locator support
> is part of the DRDA spec so could be added to network server. See
> http://www.opengroup.org/dbiop/
> It is not clear if SQL Support for LOB locators would also be required.
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