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Overviews are tables that represent a reduced version of a dataset intended for efficient rendering at certain zoom levels while preserving the general visual appearance of the complete dataset.
The reduction consists in havig a fewer number of records (while each overview record may represent an aggregation of multiple records) and/or simplified record geometries.
Overviews are created through the CDB_CreateOverviews function.
The statement timeout may need to be adjusted before using this function,
as overview creation for large tables is a time-consuming operation.
The CDB_Overviews function can be used determine what overview tables
exist for a given dataset table and which zoom levels correspond to it.
The CDB_DropOverviews function removes a dataset's existing overviews.
To know if overview tables exist for some base table, and to obtain
a list of which overview tables are approrpiate for which zoom levels,
the CDB_Overviews functions can be used.
The zoom level we're referring here to are those used by the tiler: http://wiki.openstreetmap.org/wiki/Zoom_levels
CDB_CreateOverviews
Create overviews for vector dataset.
Using the function
The table for which overviews will be generated should be a Cartodbfied dataset with vector geometry.
SELECT CDB_CreateOverviews('table_name');
--- Generates overview tables for the dataset
Arguments
CDB_CreateOverviews(table_name, ref_z_strategy, reduction_strategy)
- table_name regclass, table for which overviews will be generated
- ref_z_strategy regproc, optional function that provides
the Z-scale strategy.
It returns the base Z level for the dataset.
It should have these arguments:
- table_name regclass, table to compute the reference Z scale for
- reduction_strategy regproc, optional function that provides
the reduction strategy to generate an overview table from a table
for a smaller scale (higher Z number).
It returns the name of the generated table.
It should have these arguments:
- base_table_name regclass, base table to be reduced.
- base_z integer, base Z level assigned to the base table.
- overview_z integer, Z level for which to generate the overview.
Tolerance / level of detail
The level of detail to be representable by each overview layer can
be specified as a tolerance in pixels (if different from the default of 1 pixel)
with the function CDB_CreateOverviewsWithToleranceInPixels
which has as a second additional argument the desired tolerance.
This tolerance defines the maximum deviation in pixels of the overviews geometries with respect to the original geometries when overview tables are used for their intendend zoom level.
CDB_Overviews
Obtain overview metadata for a given table (existing overviews). The returned relation will be empty if the table has no overviews.
The function can be applied to a single table:
SELECT CDB_Overviews('table_name');
--- Return existing overview Z levels and corresponding tables
Or to multiple tables passed as an array; this can be used
to obtain the overviews that can be applied to a query by
combining it with CDB_QueryTablesText:
SELECT CDB_Overviews(CDB_QueryTablesText('SELECT * FROM table1, table2'));
--- Return existing overview Z levels and corresponding tables
The result of CDB_Overviews has three columns:
| base_table | z | overview_table |
|---|---|---|
| table1 | 1 | table1_ov1 |
| table1 | 2 | table1_ov2 |
| table1 | 4 | table1_ov4 |
| table2 | 1 | table1_ov1 |
| table2 | 2 | table1_ov2 |
Arguments
CDB_Overviews(table_name)
- table_name regclass, oid of table to obtain existing overviews for
CDB_Overviews(table_names)
- table_names regclass[], array of table oids
CDB_DropOverviews
Remove the overviews of a table, if present.
SELECT CDB_DropOverviews('table_name');
Arguments
CDB_Overviews(table_name)
- table_name regclass, table for which to drop existing overviews.