Development:XMLDB modifying DML functions
XML database schema > Roadmap > Modifying DML functions
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Intro/explnation here...
All the work will be performed in one branch, call it MOODLE_17_DB_DML. This point must be performed after Point 1: Splitting datalib.php was finished.
Details
This is a long process in which a lot of scripts become affected although critical changes are, exclusively in the DML functions. The rest is a repetitive work, changing how such functions are used in hundreds of places and testing them carefully. Documentation of the new final DML library must be ready as soon as possible to allow 3rd part developers to update their contributions.
Following, there is one list of changes to be performed in the DML library with their implications in the rest of the code (note: as the number of changes isn't too much big, move each item to one section!)
LIMIT x, y clause
We need to change all the uses of the LIMIT offset, num clause (both Oracle and MSSQL doesn't support such clause) to use the cross-db compatible SelectLimit() function. To do it, all the get_recordXXX and get_recordsetXXX functions in dmllib.php must be changed to support two more parameters $sqlnum and $sqlfrom, to specify the number of records to retrieve and the number of records to skip. This change in the library, although important, will be backwards compatible with the old Moodle code using hand-written LIMIT clasuses.
Once changed, all Moodle core should be transformed from the old LIMIT x,y uses to the new functions with the new parameters.
Also, the sql_paging_limit() will be deprecated ASAP (2.0?).
INSERT of clob/blob data
Oracle doesn't supports direct injection of clob/blob (text/binary) data to DB. Instead, a two phase transaction must be performed, first inserting the record using empty_clob() to create the container and then, updating the record with real contents with the 'UpdateXlob() functions.
This has the counterpart that we must know what fields are clob/blob, perhaps asking Metadata functions (and caching results). Then, follow the above approach for each lob column in the insert.
Note that nothing is specified in the ADOdb documentation about updating clob/blobs so they should be supported directly by the UpdateXlob() functions without problems (needs to be checked with real data).
