106 lines
3.2 KiB
Markdown
106 lines
3.2 KiB
Markdown
Overviews are tables that represent a *reduced* version of a dataset intended
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for efficient rendering at certain zoom levels while preserving the
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general visual appearance of the complete dataset.
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The *reduction* consists in a fewer number of records
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(while each overview record may represent an aggregation of multiple records)
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and/or simplified record geometries.
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Overviews are created through the `CDB_CreateOverviews`.
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The statement timeout may need to be adjusted before using this function,
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as overview creation for large tables is a time-consuming operation.
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The `CDB_Overviews` function can be used determine what overview tables
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exist for a given dataset table and which zoom levels correspond to it.
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The `CDB_DropOverviews` remove a dataset's existing overviews.
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### CDB_CreateOverviews
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Create overviews for vector dataset.
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#### Using the function
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The table for which overviews will be generated should be
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a Cartodbfied dataset with vector geometry.
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```sql
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SELECT CDB_CreateOverviews('table_name');
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--- Generates overview tables for the dataset
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```
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#### Arguments
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CDB_CreateOverviews(table_name, ref_z_strategy, reduction_strategy)
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* **table_name** regclass, table for which overviews will be generated
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* **ref_z_strategy** regproc, optional function that provides
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the Z-scale strategy.
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It returns the base Z level for the dataset.
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It should have these arguments:
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- **table_name** regclass, table to compute the reference Z scale for
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* **reduction_strategy** regproc, optional function that provides
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the reduction strategy to generate an overview table from a table
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for a smaller scale (higher Z number).
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It returns the name of the generated table.
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It should have these arguments:
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- **base_table_name** regclass, base table to be reduced.
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- **base_z** integer, base Z level assigned to the base table.
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- **overview_z** integer, Z level for which to generate the overview.
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### CDB_Overviews
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Obtain overview metadata for a given table (existing overviews).
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The returned relation will be empty if the table has no overviews.
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The function can be applied to a single table:
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```sql
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SELECT CDB_Overviews('table_name');
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--- Return existing overview Z levels and corresponding tables
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```
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Or to multiple tables passed as an array; this can be used
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to obtain the overviews that can be applied to a query by
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combining it with `CDB_QueryTablesText`:
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```sql
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SELECT CDB_Overviews(CDB_QueryTablesText('SELECT * FROM table1, table2'));
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--- Return existing overview Z levels and corresponding tables
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```
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The result of `CDB_Overviews` has three columns:
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| base_table | z | overview_table |
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|------------+---+----------------|
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| table1 | 1 | table1_ov1 |
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| table1 | 2 | table1_ov2 |
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| table1 | 4 | table1_ov4 |
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| table2 | 1 | table1_ov1 |
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| table2 | 2 | table1_ov2 |
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#### Arguments
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CDB_Overviews(table_name)
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* **table_name** regclass, oid of table to obtain existing overviews for
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CDB_Overviews(table_names)
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* **table_names** regclass[], array of table oids
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### CDB_DropOverviews
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Remove the overviews of a table, if present.
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```sql
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SELECT CDB_DropOverviews('table_name');
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```
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#### Arguments
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CDB_Overviews(table_name)
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* **table_name** regclass, table for which to drop existing overviews.
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