Symbian Database Components

Other Points

Since most of the calls to EDBMS are asynchronous, typically the developer makes the EDBMS call from within a client-side active object. Using that combination avoids causing applications to block due to long-running database operations. However, if for some reason you are not using active objects client-side, or in other cases, sometimes the RDbIncremental class can be useful. This lets clients do things such as create an index in steps. Instead of having to wait a long time while an index is being built, for example, client-side programs can start the index creation process at one point, then later on ask the EDBMS server to do a little more work on the index creation, and so on.

Difference Between RDbView and RDbTable

Again, RDbView is not a normal database view; on the contrary, it is a client-side rowset. For certain queries, there's a more efficient client-side rowset structure: RDbTable. This is another rowset structure--the efficiency is server-side. Whereas in the RDbView example, the internal server-side result structure is actually a dynaset, a separate server-side copy of the data. When using RDbTable, the server-side structure is not a dynaset, but instead has pointers referring directly to the table structures.

Modifying Data

Thinking through the types of applications that use the Symbian DBMS gives the conclusion that updates beyond the simple functionality are not high on the priority list, and the Symbian implementation is no exception. The SQL update is somewhat limited, allowing non-joining where clauses, but not complex expressions in the SET portion of the update statement. To update a column value with a complex expression, the expression calculation would have to be done outside of SQL.

Developers who choose not to use SQL to modify table data are required to:

1. Create a rowset.
2. Update the rowset.
3. Indicate the updating.

Using SQL as much as possible requires much less programming and is a good example of why I recommend using SQL.

Deleting Data

 
delete from authors where author_id = 0

Indexes

SymbianOS supports multiple indexes on multiple columns on a table, and indexes can be unique. Listing Five is Symbian SQL for creating two multiple-column indexes on the same table.

create unique index id_index on authors(author_id)
   create index index2 on authors(last_name, first_name)
   As you can see, this is straightforward SQL.

Datatypes

Symbian's DBMS server supports a rich variety of datatypes, from bits, tinyints to smallints, integer, bigint, as well as real, float, and double precision. Auto-increment columns are supported as well. There is a date type, a time type, and a timestamp, char(n), varchar(n), binary(n), and varbinary(n).

Client-side Access

If a program will be the exclusive user of a database, it is possible to bypass the DBMS server layer by using client-side access. In that case the client program directly accesses the system file server which is inside the base of the OS, and data retrieval will be faster than in the case where server. This is done by opening the database using the RDbStoreDatabase structure.

Conclusion

As mobile devices get smarter and smarter, there will be continue to be a wide variety of choices for deploying databases. Obviously, server-based databases (where the mobile device stores data on some large server machine to which the mobile device has some connection) should be given a hard look. If that is impractical, consider some local alternatives. A local alternative, in contrast to storing all data on a server, involves storing some or all the application data on the mobile device. A full-featured relational database implementation packs a lot of functionality but adds to the footprint on each device, and can require royalties and/or licenses per device. Since the internal Symbian DBMS is part of the OS, there's no per-device royalty. The Symbian DBMS implementation is a powerful implementation given its footprint--clearly it works for some applications, evidenced by Symbian's use of its DBMS for standard Symbian system applications such as Contacts, the Symbian standard application for phone contacts. In addition to internal use, and system applications, for some third-party applications using an all-local, or temp-local and replicate strategy, Symbian's royalty-free, small footprint, provided DBMS, makes sense as well.