[fdo-internals] Use of SQL pass through

Haris Kurtagic haris at sl-king.com
Thu Jun 4 05:41:14 EDT 2009

I would like to divide this and answer in two parts.


Part with Parameters direction is something we need. As it is already
written , mostly to be able to execute and get results from stored


About executing SQL statements and trying to squeeze result of it inside
FDO class/schema, I think it is too complex and in my mind without
chance to be successful. There is so many cases in which it can't be
done properly. If we are missing some info about result of execution of
SQL we could look into existing specs like ODBC and add those. I agree
FDO application should be able to get all necessary info about executed
SQL so app can be written in generic way but I don't see putting that
info in FDO class or reengineering sql etc.. I believe what can be done
with api's like odbc is ok for fdo api too.




From: fdo-internals-bounces at lists.osgeo.org
[mailto:fdo-internals-bounces at lists.osgeo.org] On Behalf Of Orest
Sent: Wednesday, June 03, 2009 3:28 PM
To: FDO Internals Mail List
Subject: [fdo-internals] Use of SQL pass through




I would like to solicit feedback, discussion, and suggestions on
requirements and issues that we are seeing with the FdoISQLCommand which
is used for the purpose of SQL pass through for some of the RDBMS-based


Please see the discussion below.







Enhanced SQL Command Support Discussion


The FDO API currently defines support for a SQL command that allows for
pass-through execution of SQL statements either through a non-query
execution of the SQL statement directly in the underlying Data Store, or
through a query mechanism that returns a simple data reader listing the
properties returned as a result of the SQL execution. The definition of
the SQL command has remained fairly static over the last number of
releases as primary development efforts have focused on extending other
aspects of the FDO API, implementing new providers, etc. However,
requirements for change to the SQL command have accumulated for us as
RDBMS providers have implemented SQL pass through support and clients
have attempted to integrate use SQL pass through into their applications
in a seamless manner. While we generally expect applications to be using
the generic FDO Select and other commands using FDO expressions,
application still need to execute direct SQL against RDBMS-based
providers on an exception basis for things that cannot do through FDO.
While we hope to improve the FDO expression capabilities over time,
there will always be this need for direct SQL processing for exception

This email is meant to start discussion on how to handle the
requirements that we've been seeing and to get feedback on how to modify
the FDO api to handle these requirements. We have some ideas as
described below, but would like to get other feedback. We'll draft an
RFC once we get close to a consensus on how it should look.


One key request has been the desire to have the FDO API 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. 

Other more SQL specific requirements related to this have also arisen,

*	A need to allow a client to set the size of the fetch array an
FDO provider will use when executing SQL statements ([OH] I still have
an issue with this one. Fetch sizes are internal tuning parameters and
are not FDO api concepts. Other providers that deal with select or
insert buffering have default internal sizes.)
*	A need to specify bind variables for the SQL command, including
arrays of bind variables. Since SQL commands may include stored
procedure execution, bind variables need indication of whether they are
input, output, or return parameters.


In general, the intent of SQL pass-through is to deal with physical
schemas. There is no parsing of the SQL statements, and what are
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 the mixing of 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
either of 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. 

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 this 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 FDO schema and class definitions. At this
point no standards exist for naming auto-generated schema, class and
property names. It would be beneficial if, as a result of this RFC, some
uniform naming 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: 

*	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
*	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.


In order to support the SQL pass-through objective outlined above, the
FDO API will need to be modified to provide a new capability function to
state that this is supported, and to provide a way to return an explicit
FDO Feature Reader. Two possibilities have been identified for returning
the feature reader. One idea is to add a new ExecuteFeatureReader (or
similar name) method to FdoISQLCommand. The existing ExecuteReader
method will remain as is for backwards compatibility. Another
possibility is to keep the SQL pass through interface unchanged but
provide a utility that will convert the returned SQL Data Reader into a
Feature Reader after the fact.

A related question is around the class definition that would be
generated from the SQL since it often would be a temporary class
definition for the command and not something found from the FDO Describe
Schema command. The feature reader would expose that class definition,
but would applications need to get that earlier, e.g. with a new method
on FdoISQLCommand to describe the generated class definition prior to
getting the reader?

Parameter Direction

FDO already includes API's for defining and using parameters (bind
variables). However, there is no indication of direction (input versus
output) and if an application is planning to use stored procedure calls
in the SQL pass through, they would need to provide some indication of
input, output, or return parameters. A possibility is to add a new
FdoParameterDirection enumeration and add set/get methods to the
FdoParameterValue or FdoParameterValueCollection interface.

A related question is whether FdoISQLCommand needs the application to
tell it whether the SQL command being executed is a stored procedure
call or some other type of SQL? We want to avoid having providers be
forced to parse SQL where currently they probably all just send the SQL
directly to the underlying server. The ExecuteNonQuery versus
ExecuteReader methods that the application uses already tell the
provider whether to expect results back from the SQL or not. A stored
procedure call is a "non-query" but can return results, so is it another



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