Add function to obtain existing overviews of a table

doc/CDB_Overviews.md

@@ -1,3 +1,19 @@
+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 `CDB_Overviews` function can be used determine what overview tables
+exist for a given dataset table and which zoom levels correspond tt
+
+
+### CDB_CreateOverviews
+
 Create overviews for vector dataset.
 
 #### Using the function
@@ -26,3 +42,18 @@ CDB_CreateOverviews(table_name, ref_z_strategy, reduction_strategy)
   - **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)
+
+```sql
+SELECT CDB_Overviews('table_name');
+--- Return existing overview Z levels and corresponding tables
+```
+
+#### Arguments
+
+CDB_Overviews(table_name)
+
+* **table_name** regclass, table for which overviews will be generated