On 23 September 2013 22:10, Alexey Botchkov <holyfoot@askmonty.org> 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

my_bool msudf_within_init(UDF_INIT *initid,UDF_ARGS *args,char *message)
...
	args->arg_type[0] = STRING_RESULT;
...

long long msudf_within(UDF_INIT *initid,UDF_ARGS *args,char *is_null, char *error)
...

geom1 = msudf_getGeometry((unsigned char *)args->args[0],args->lengths[0]);		 

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...

GEOSGeom msudf_getGeometry(unsigned char *buf,unsigned int length)
{
	int srid;
	GEOSGeom geom;


	geom = GEOSGeomFromWKB_buf(buf + 4,length - 4);
	if (geom != NULL) {
		srid = msudf_getInt(buf);
		GEOSSetSRID(geom,srid);
	}
	return geom;
}

int msudf_getInt(const unsigned char *buf)
{
	int result;
    unsigned char *p = (unsigned char *)&result;

    p[0] = buf[0];
    p[1] = buf[1];
    p[2] = buf[2];
    p[3] = buf[3];

	return result;
}

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

MSUDF_API char*
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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