[gdal-dev] Questions about SQL cursors in GDAL driver

[Tamas Szekeres]

2009/7/11 Frank Warmerdam <warmerdam at pobox.com>

> Jorge Arévalo wrote:
>
>> Hello,
>>
>> 2009/7/10 Tamas Szekeres <szekerest at gmail.com>:
>>
>>> Jorge,
>>>
>>> I'm a bit hesitant to think using a server cursor would be the best
>>> approach
>>> here. Could you explain why is this needed?
>>>
>>
>> Yes. As now I'm working basically with regularly tiled coverages, an
>> operation like SELECT rast FROM table gives me all the tiles. These
>> tiles don't overlap, and its size can be the size of a block read for
>> IReadBlock method. So, if I have a cursor, I can fetch the tile that
>> the IRasterBlock needs as one of the rows of the resultset without
>> having all the tiles loaded in memory at same time.
>>
>
> Jorge,
>
> I'm not clear on how you know what row to take out of the resultset
> for a given IReadBlock() call?  How do you translate the x/y block
> index into a resultset row offset?
>
> I would note that spatial searches *should* be quite fast if the
> raster row is spatially indexed.
>

Jorge,

I'm thinking of exactly the same as Frank. It's ok you have the same block
size in GDAL as the block size in the database but you might have to
construct a different SQL query to select the corresponding block even if a
single row is retrieved in a single IReadBlock call.

Best regards,

Tamas
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