Dropping raw data

Note that if any still-refreshing (more recent than start_offset) part of the continuous aggregate is dropped via a retention policy or direct drop_chunks call, the aggregate will be updated to reflect the loss of data. For this reason, if it is desired to retain the continuous aggregate after dropping the underlying data, the start_offset of the aggregate policy must be set to a smaller interval than the drop_after parameter of a hypertable's retention policy. Similiarly, when calling drop_chunks, extra care should also be taken to ensure that any such chunks are not within the refresh window of a continuous aggregate that still needs the data. More detail and examples of this can be seen in the the data retention documentation.

This is also a consideration when manually refreshing a continuous aggregate. Calling refresh_continuous_aggregate on a region containing dropped chunks will recalculate the aggregate without the dropped data. This can lead to undesirable results, such as replacing previous aggregate data with NULL values, given that the raw data has subsequently been dropped.

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