Monday, 7 January 2013

Version 4.8 released

This brings standard SQL diagnostics management. Standard SQL exception handling includes three kinds of handlers for errors: CONTINUE, EXIT and UNDO. Pyrrho honours not_found if it is handled but otherwise does not report an error condition if DML operations do not affect any rows. Pyrrho's strict interpretation of correct transaction behaviour is supported by better prompts in the command line interface and a prohibition on using COMMIT inside a SQL routine (ROLLBACK is allowed but aborts the routine). The meaning of UNDO is very interesting: if this handler is activated, all of the database changes performed so far in the scope of the handler declaration are undone. This is a great improvement on ROLLBACK or EXIT as it allows for subtle and sophisticated error handling. Exceptions arising from most of Pyrrho's SQLSTATEs can be handled: you cannot handle such errors as damaged files, bad syntax, or attempts to defeat the transaction mechanism. In addition the keywords not_found and sqlexception etc are supported, and it is permissible to use your own signals (the usual rules for identifiers apply). The diagnostics area in the SQL standard is interesting. There is a set of keywords, including message_text, some of which have prescribed default values for each exception. You can override these using SET assignments in a SIGNAL statement, and collect their values by using GET DIAGNOSTICS, which allows assignment of such data identified by keywords to variables that you specify. In addition, a roundup of improvements results in better behaviour of such things as structured types. The philosophy nowadays is that client software should not need to know about how data types are implemented in the server, but that (as in XML or JSON) should simply give a full and readable description of structured data. The Pyrrho engine uses strong data types, so the server describes each part of the data by giving the nominal data type and the actual data subtype if it is different. For example, the actual type might include semantic information or metadata. Such actual types are visible to SQL and the user when they are explicitly assigned to the value using the RDF/OWL ^^ suffix or the SQL TREAT function. Unfortunately some previous cross-platform support for other APIs is now lost, although many of the interfaces in ADO.NET and JPA are still supported. For example, there seems to be no way for .NET's DataTable to support (nested) structured types, and so DataAdapters and some of the more complex JPA interfaces have been dropped in this edition. The venerable PyrrhoMgr tool is a casualty of this change. I still rather like the PyrrhoSQL tool but it needs someone other than me to improve its usability.

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