Note that if any still-refreshing (more recent than
start_offset) part of the
continuous aggregate is dropped via a retention policy or
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
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.
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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