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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 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.
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 remove a dataset's existing overviews.
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.
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.