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Re: [Spatial] On current implementation approach


On 23 September 2013 22:10, Alexey Botchkov <holyfoot@xxxxxxxxxxxx> wrote:
>> 1. Is it possible to implement MariaDB extensions like Spatial (custom
>> type + set of functions) without such a tight coupling with the
>> internal implementation of the type system (without messing Field
>> class with geometry types directly, etc.)?
> Yes, it is possible. The core algorithms are separated from the Field
> structure and any other database internals.
> They are placed in sql/gcalc_slicescan.cc and sql/gcalc_tools.cc files.

Yes, but my question is not really about location of computational geometry
bits, but about the data management: SQL data type for geometry objects,
input/output routines.

Due to my lack of experience with MariaDB/MySQL UDF, I simply assumed that if:
1. Field is the only place that defines GEOMETRY type (and there is no
CREATE TYPE support)
2. UDF prototypes will use of GEOMETRY in their prototypes to declare
input/output parameters
then I couldn't understand how it is possible to remove geometry
definitions from Field
and other internal definitions.

But, I've just found this project [1] with extra spatial UDFs, so I
think I understand the UDF
protocol regarding I/O arguments would not require explicit GEOMETRY type
making it possible to move Spatial Extensions completely out of
built-ins (trunk/sql/ files).

[1] https://github.com/krandalf75/MySQL-Spatial-UDF

>> 2. Is it possible to implement Spatial using User-Defined Functions
>> (UDF) defined in shared binary?
> The spatial functions/operations can be implemented with UDF, but
> that makes query optimization and using Spatial keys problemmatic.

So, for real use case, the idea I brainstormed above would not make sense.
Unless, there is workaround for those problems you mean.

>> 3. What is the reason behind using Well-Known-Binary (WKB) stream of
>> bytes to transport geometry values into/from functions? Is it due to
>> limitations of MariaDB type system where String is  the only universal
>> carrier for complex data? This concern is related to necessity of
>> encoding/decoding WKB when chaining spatial function calls, and
>> possibilities to avoid it.
> The reason was mostly historical. It was sufficient for the first
> implementations of the Geometry field types and somewhat convenient as
> we don't need to perform conversions
> when we need to import/export features in their WKB representation.
> But yes, that format is inefficient and difficult to handle properly. I plan
> to get rid of it internally - only support importing-exporting it.

I roughly understand, but how do you plan to pass geometry data around,
in what format?

AFAIU, it is not possible to pass user-defined types into/from SQL functions,
so geometries would have to be passed as String objects anyway, wouldn't they?
IOW, there are only 3 types available (integer, real, string), so
String is the only one
usable to pass geometry objects around, regardless of actual encoding format,
WKB, WKT, any other binary stream...

It means, that if I want to pass geometry to my_foo UDF:

my_foo(UDF_INIT *initid,UDF_ARGS *args, char *buf, unsigned long
*length, char *is_null, char *error);

the only option available is to make geometry into a kind of stream of bytes
and passed as one of args item.
So, a kind of serialising/deserialising is in fact unavoidable.

Is my understanding correct?

Best regards,
Mateusz  Loskot, http://mateusz.loskot.net
"Participation in this whole process is a form of torture" ~~ Szalony

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