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Overviews documentation fixes

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This commit is contained in:
Javier Goizueta 2016-04-27 16:23:25 +02:00
parent 1358964628
commit 3e34ca4654
1 changed files with 16 additions and 5 deletions
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21
doc/CDB_Overviews.md
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@ -2,18 +2,25 @@ 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
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`.
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` remove a dataset's existing overviews.
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
@ -51,10 +58,14 @@ CDB_CreateOverviews(table_name, ref_z_strategy, reduction_strategy)
#### 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 2 pixels)
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).
@ -79,7 +90,7 @@ SELECT CDB_Overviews(CDB_QueryTablesText('SELECT * FROM table1, table2'));
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 |