Executing Statements

Once the creation stage has been completed, Virtual DataPort is ready to accept queries and/or updates (updates can only be executed on “updateable views” as defined by the SQL 92 standard). Details on how external applications can send and execute statements on the Virtual Database are provided in the Developer Guide. This section provides only a general overview of the statement execution process from an internal point of view.

When a VQL query is received, the Virtual DataPort query interpreter starts by checking if the query capabilities of the views involved allow obtaining an answer to the query. If the query cannot be answered to, the user is informed of this. If it can, the process continues.

Subsequently, the Plan Generator creates the possible execution plans for the query. The plans normally differ in aspects such as the algorithm used to execute the joins or the specific search methods selected on the sources.

The optimizer module is responsible for obtaining the cost of each of the plans, according to different parameters, so that the best can be selected. This process, among other tasks, is responsible for optimally distributing processing between the Virtual DataPort server and the sources, delegating operations such as groupby, selection conditions, joins or unions, where possible. Hence, data transfer on the network can be minimized. This stage is also responsible for tasks such as choosing the most suitable method for implementing join operators, for establishing the swapping to disk strategy for very large results or for managing use of the cache module. See the section Optimizing Queries of the Administration Guide for more details.

Once the optimum plan has been selected, the Execution Engine puts it into practice. Execution of a plan assumes execution of a series of sub-queries expressed in terms of the base relations only. These sub-queries will be executed by the wrapper of the corresponding source. It is remarkable how Virtual DataPort is capable of making maximum use of parallelism, whereby sub-queries are normally executed in parallel.

Finally, the execution engine combines the results returned by the sources in accordance with that specified for each plan, thus obtaining the final response to the query.

It is important to highlight that the system operates in an asynchronous manner. This means that as the results of the sources become available, the system begins to process them even if the sources have not yet issued a complete response. This considerably speeds up the times for obtaining the first tuples of the final result. Another important aspect is that the system is capable of processing partial results, i.e. it can process the query even if some of the sources are temporarily inaccessible, providing the results that can be obtained with the remaining sources while still informing the client application of the errors.

Another fundamental aspect is that, optionally, as mentioned previously, all or part of the source data can be pre-loaded in the server cache. In this case, the system will check if the sub-query received can be resolved with the data contained in the cache. If this is the case, the response is obtained directly from same instead of querying the source.

Virtual DataPort also supports the executing of updating statements (INSERT / UPDATE / DELETE) on views, provided these can be updated according to the standard definition in SQL-92. See section Inserts, Updates and Deletes Over Views for further details.