On Tue, Jan 11, 2005 at 05:28:23PM -0000, Mark Cave-Ayland wrote:
> Hi strk,
>
> > -----Original Message-----
> > From: strk at refractions.net [mailto:strk at refractions.net]
> > Sent: 11 January 2005 15:17
> > To: Mark Cave-Ayland
> > Cc: 'PostGIS Development Discussion'
> > Subject: Re: [postgis-devel] PATCH: New positional operators
> > based on Y position of bounding boxes
>
> (cut)
>
> >
> > Thanks. I've deleted lwgeom/lwgeom.sql.in from the
> > repository. It is no more maintained nor used since a long
> > time (I'm sorry
> > you lost time modifying it as well).
>
> OK, I wasn't sure if it was some sort of low-level testing script so I
> updated it anyway :)
>
> > I'm really sorry about it, but - yes - it's true.
> > We should remove all 71 special handling around.
>
> I think it would be good to do this sort of housekeeping as we go along, as
> it will help keep the code a bit tidier and more understandable.
Ok, let's get rid of them ...
>
> > Added SRID checks.
>
> Thanks :)
>
> > Sounds hard to me... how can you binary search a space with
> > unknown boundaries ? Anyway, now that you make me think about
> > this I implemented btree index on geometry for use in ORDER
> > BY and GROUP BY. geom1 < geom2 if geom1.bbox.xmin <
> > geom2.bbox.xmax and consequently other geoms too... I think
> > it would be easier that way (but you need to build a btree index).
>
> Well, I've been working on some code that opens the GiST index relation
> directly and reads in the root node to calculate the extents of the root
> node. Of course this does not reflect the true extents since we do not know
> the visibility of each of the index entries without reading the tuples
> themselves; however it is a very good starting point. Here is the code that
> I have experimenting with (add to the end of lwgeom_gist.c):
Wow.. I've been trying myself but didn't get feedback from pgsql-hackers
and coudln't find documentation myself. Sounds good and simpler then
I tought...
Anyway, consider also the already present estimated_extent(), returning
the extent of the computed histogram (but only works on 800).
BTW, which pgsql versions will support your GiST root extractor ?
--strk;
>
> mca1=# select index_extent('t1_idx');
> index_extent
> --------------
> BOX(0 0,0 2)
> (1 row)
>
>
> // Index extent function
>
> static char
> *t2c(text* in) {
> char *out=palloc( VARSIZE(in) );
> memcpy(out, VARDATA(in), VARSIZE(in)-VARHDRSZ);
> out[ VARSIZE(in)-VARHDRSZ ] ='\0';
> return out;
> }
>
> /*
>
>
> index_extent takes one parameter: the name of the GiST index as a string
>
>
> CREATE OR REPLACE FUNCTION index_extent(text)
> RETURNS box2d
> AS '$libdir/liblwgeom.so.1.0','index_extent'
> LANGUAGE 'C' WITH (isstrict,iscachable);
>
> */
> //
>
> #include "access/genam.h"
> #include "storage/lmgr.h"
> #include "utils/builtins.h"
> #include "catalog/namespace.h"
>
> Datum index_extent(PG_FUNCTION_ARGS);
> PG_FUNCTION_INFO_V1(index_extent);
> Datum index_extent(PG_FUNCTION_ARGS)
> {
> text *name=PG_GETARG_TEXT_P(0);
> char *relname=t2c(name);
> List *relname_list;
> RangeVar *relvar;
> Relation index;
> Buffer buffer;
> Page page;
> ItemId iid;
> BOX2DFLOAT4 *item, *extent;
> OffsetNumber i, maxoff;
>
>
> // Open the passed index
> relname_list = stringToQualifiedNameList(relname, "postgis");
> relvar = makeRangeVarFromNameList(relname_list);
> index = index_openrv(relvar);
> PG_FREE_IF_COPY(name,0);
> LockRelation(index, AccessExclusiveLock);
>
> // Read the root node of the GiST index, maxoff is the number of
> entries
> // in the root node
> buffer = ReadBuffer(index, GISTP_ROOT);
> page = (Page) BufferGetPage(buffer);
> maxoff = PageGetMaxOffsetNumber(page);
>
> // Cycle through each of the entries in the root node
> extent = palloc0(sizeof(BOX2DFLOAT4));
> for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
> {
> iid = PageGetItemId(page, i);
> item = (BOX2DFLOAT4 *) PageGetItem(page, iid);
>
> // Store the largest extents to represent the bounding box
> of the root node
> if (item->xmin < extent->xmin)
> extent->xmin = item->xmin;
> if (item->xmax > extent->xmax)
> extent->xmax = item->xmax;
> if (item->ymin < extent->ymin)
> extent->ymin = item->ymin;
> if (item->ymax > extent->ymax)
> extent->ymax = item->ymax;
> }
>
> // Close the passed index
> UnlockRelation(index, AccessExclusiveLock);
>
> ReleaseBuffer(buffer);
> index_close(index);
> pfree(relname);
>
> // Return the final extent as a BOX2D
> return PointerGetDatum(extent);
> }
>
>
> I'm not sure whether we can use the btree operator class to quickly return
> the extents as at the moment I believe the difference metric is area?
> However, this is really just me experimenting to see whether we can maximise
> the information from the GiST index to calculate the extent more quickly :)
>
>
> Kind regards,
>
> Mark.
>
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