= FDO RFC 33 - SQL Pass-Through Feature Reader Support =

This page contains an change request (RFC) for the FDO Open Source project.  
More FDO RFCs can be found on the [wiki:FDORfcs RFCs] page.


== Status == 
 
||RFC Template Version||(1.0)|| 
||Submission Date|| April 9, 2009 ||
||Last Modified|| Greg Boone [[Timestamp]]||
||Author||Greg Boone, Orest Halustchak||
||RFC Status||Not Ready|| 
||Implementation Status||Pending||
||Proposed Milestone||3.5.0.0||
||Assigned PSC guide(s)||Greg Boone||
||'''Voting History'''||(vote date)||
||+1|| ||
||+0|| ||
||-0|| ||
||-1|| ||

== Motivation ==

Enable the FDO API to support SQL pass-through commands that return an FDO feature reader, referencing a proper FDO schema, not simply an FDO data reader. The feature reader will contain proper geometry properties, relations and associations. This enhancement is also intended to allow client applications that use FDO Feature Readers to code their applications in a generic manner so that data coming back from Select or SQL Pass Through statements can be processed in a uniform manner, thus reducing complexity, costs and time to implement. 

== Overview ==

In general, the intent of SQL pass-through is to deal with physical schemas. There is no parsing of the SQL statements, and what is identified are physical schema tables, columns, functions, commands, etc. The SQL statement can be any SQL command, not just select, but any DML (select, insert, update, delete) or any DDL (create, drop, alter, …). However, FDO feature readers deal with FDO logical schemas, which is at a different conceptual level. It’s mixing these levels that causes difficulty for applications using the FDO API. Applications are required to use different code pathways to handle select statement as opposed to direct SQL execution. If clients could use the result of both these types of operations in a generic routine or component, such as a Data Table or Symbolization packages, applications would be provided a much more seamless and dynamic mechanism on which they can build and extend their applications. 

To a certain degree, the current FDO feature reader implementation assumes an FDO class definition is directly related to the properties being returned. With physical schema SQL (let’s say just Select command), there isn’t necessarily any FDO class definition that applies. This is why currently the SQL command's Execute method returns a data reader, which handles any generically returned data. Note that the FDO select aggregates command doesn’t return a feature reader either, since it’s returning computed data. 

One response to this issue has been to suggest that providers reverse engineer the select results and attempt to find the FDO class definition (coming from a describe schema request) that matches it. Other suggestions have been to construct a class definition on the fly, one that would not result from a call to describe schema. If the select is from a single table, providers would find the class definition that is based on that table and use it. However, then matching up the columns being selected with the properties in the logical schema, there may be some slight differences. This would result in the mixing of physical schema and logical schema elements together. For example, class names that are not the same as table names, property names that are not the same as column names, the use of additional computed columns, inclusion of pkey columns, etc. Granted that in many cases, the logical and physical views would be virtually identical. However, since that cannot be guaranteed, the design that is adopted will have to be able to handle the general case. 

To account for these scenarios, providers will need to be modified to reverse engineer class definitions from the selected data and not attempt to match the select request to an existing FDO class. IN reality, FDO Providers will be required to handle this in any case since a generic SQL select may not match up at all with an existing FDO class. An example of this can be found when selecting from a table with an owner that is different from the connected data store. For example, Connect to data store called Denver and select from Boulder.Roads – the schema may or may not be similar to Denver.Roads. In these types of circumstances, providers should manufacture a new class definition unless they could determine with certainty that the select is on a table that matches one of the already defined FDO classes.

In cases where the resulting columns come from an existing FDO feature table, a provider can return the class definition corresponding to that table. In cases where the columns come from an unknown table, a class definition can be constructed on the fly. By definition, the FDO class definition returned by a feature reader does not necessarily correspond exactly to an existing FDO class definition. Existing class definitions may contain the properties that were asked for in the Select command, plus additional computed properties. It is perfectly legal to return a constructed class definition, which is only valid for the select that was executed, and not usable for further updates or inserts.

In the circumstance that a computed class is generated, the FDO class definition’s !IsComputed property will return true. In that manner, applications are able to distinguish the content of the feature reader responses coming from the providers and tailor their implementation accordingly. In such a situation, some care will also need to be given to the name of the generated schema and classes. At this point no standards exist for naming auto-generated or comuted schema, class and property names. It would be beneficial if, as a result of this RFC, some uniform conventions could be adopted. 

Providers that do return feature readers from SQL commands will need to come up with the appropriate class definition that the feature reader could expose. Here are a few general use cases that can be used to guide their implementations:

    - Select is against a table that has an existing class definition and the select returns the same information as defined by the class definition (e.g. select * from roads).
    - Select is against a table that has an existing class definition, but the select returns other information such as a subset of properties or additional computed properties. The FDO select command handles this type of case as well. The class definition returned by the feature reader includes the specific properties for that select.
    - Select is against a table that doesn’t have an existing class definition. Providers will be required to generate a temporary class definition.
    - Select is against a table that has an existing class definition, but the select is an aggregation that doesn’t return actual objects (e.g. select count(*), max(length) from roads). In this case it doesn’t make sense for the provider to return the same class name as for the underlying table since it is not actually returning road objects at all. This is basically another temporary class definition.

TODO: 

-- Figure out how providers can determine what are input/output parameters[[BR]]
-- Procedure calls, input/output parameters, etc....[[BR]]
-- Examples[[BR]]

== Requirements ==

=== FDO API ===

In order to support the SQL pass-through objective outlined above, the FDO API will need to be modified to provide new functions to state that this support is enabled, and return an explicit FDO Feature Reader. It is preferential to add these new methods, as opposed to editing existing functionality, so that currently designed and running applications can continue to work as designed and implemented.

==== Command Capabilities ====

An objective of this proposal is allow Provider the option to support SQL pass-through commands that return an FDO feature reader. If a provider decides to support this functionality, it can do so using the SupportsFeatureReaderFromSQLCommand method on the FdoICommandCapabilities interface. 

{{{
/// \brief
/// The FdoICommandCapabilities interface declares the feature 
/// provider's level of support for Commands.
class FdoICommandCapabilities : public FdoIDisposable
{
public:
...
...
...

    /// \brief
    /// Determines if the provider supports the ExecuteFeatureReader 
    /// function on the SQL Command. 
    /// 
    /// \return
    /// Returns true if the provider supports ExecuteFeatureReader.
    FDO_API virtual bool SupportsFeatureReaderFromSQLCommand();
};
}}}


==== SQL Command Interface ====

In order to allow Providers to return an FDO feature reader from the FDO SQL command, a new !ExecuteFeatureReader method will be added to the FdoISQLCommand interface. Providers that implement this level of functionality will need to implement this command and provide support for the use cases (which are not yet a definative list) listed above.

{{{
/// \brief
/// The FdoISQLCommand interface defines the SQL command, which 
/// supports the execution of a SQL statement against an underlying 
/// RDBMS. Three execute methods are provided to distinguish between 
/// statements that return data versus those that execute 
/// non query type operations.
class FdoISQLCommand : public FdoICommand
{
public:
...
...
...

    /// \brief
    /// Executes the SQL statement against the connection object and returns
    /// a Feature Reader exposing the physical data returned by the reader 
    /// as FDO features. The Feature Reader may return a computed class that 
    /// maps the physical schema elements, referenced by the execute, to a logical
    /// FDO schema. 
    /// 
    /// \return
    /// Returns an FDO feature reader referncing a computed class that 
    /// maps the physical schema elements, referenced by the execute, to a logical
    /// FDO schema.
    /// 
    FDO_API virtual FdoIFeatureReader* ExecuteFeatureReader();
};
}}}

== Provider Implementation ==

The question concerning which providers will need to be modified to support the SQL pass through enhancement is an open one and will depend on available development resources. I am fairly confident SQLite will be enahced to add the additional SQL Pass Through support. Adding support to SQL Server Spatial should also be high on the priority list. However, depending on resourcing, this may not be achievable in FDO 3.5.0. Support for ODBC and MySQL are considered nice to have and will be implemented if resourcing and funding can be obtained.

== Test Plan ==

Existing FDO Core unit tests will be expanded to test the proposed enhancements defined above. 

Provider specific unit tests will be added to test the proposed enhancements defined above. 

== Funding/Resources ==

Autodesk to provide resources / funding