60 lines
1.9 KiB
Markdown
60 lines
1.9 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 `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 tt
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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, function that provides 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, function that provides the reduction strategy
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to generate an overview table from a table 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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```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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#### Arguments
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CDB_Overviews(table_name)
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* **table_name** regclass, table for which overviews will be generated
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