PostGIS: use ST_EstimatedExtent instead of ST_Extent when possible

[gabrimonfa]

In PostGIS driver function GetExtent uses PostGIS function ST_Extent function.

When bForce is false it may be worth to attempt to use ST_EstimatedExtent that is much quicker and returns an extimated extent based on table metadata.

Some notes

• as of now ST_Extent support more data type than ST_EstimatedExtent
• the name has changed between PostGIS 2.0.x (ST_Estimated_Extent) and PostGIS 2.1 (ST_EstimatedExtent)
• In case of empty table or no statistics ST_EstimatedExtent returns NULL (PostGIS version >=1.5.4) or an exception. In this case we can fallback on ST_Extent

A very rough patch, to be reworked and tested, is provided

[gabrimonfa]

Rough patch to attempt to use ST_EstimatedExtent (to be reworked)

[Even Rouault]

There were several issues in your patch. The main one being that estimated_extent syntax is "select estimated_extent('schema_name', 'table_name', 'geometry_name)'" (without any from clause).

Hopefully fixed by :

trunk r27106 "PG: use ST_Estimated_Extent() on table layers if GetExtent() is called with force = 0 (derived from patch by gabrimonfa, #5427)"