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Add description about how to disable or enable IVM
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Yugo Nagata 2022-06-22 16:41:33 +09:00 committed by GitHub
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@ -98,6 +98,7 @@ refresh_immv(immv_name text, with_data bool) RETURNS bigint
```
`refresh_immv` completely replaces the contents of an IMMV as `REFRESH MATERIALIZED VIEW` command does for a materialized view. To execute this function you must be the owner of the IMMV. The old contents are discarded.
The with_data flag is corresponding to `WITH [NO] DATA` option of REFRESH MATERIALIZED VIEW` command. If with_data is true, the backing query is executed to provide the new data, and if the IMMV is unpopulated, triggers for maintaining the view are created. If with_data is false, no new data is generated and the IMMV become unpopulated, and the triggers are dropped from the IMMV. Note that unpopulated IMMV is still scannable although the result is empty. This behaviour may be changed in future to raise an error when an unpopulated IMMV is scanned.
@ -223,6 +224,12 @@ Suppose an IMMV is defined on two base tables and each table was modified in dif
If some base tables have row level security policy, rows that are not visible to the materialized view's owner are excluded from the result. In addition, such rows are excluded as well when views are incrementally maintained. However, if a new policy is defined or policies are changed after the materialized view was created, the new policy will not be applied to the view contents. To apply the new policy, you need to recreate IMMV.
### How to Disable or Enable Immediate Maintenance
IVM is effective when we want to keep an IMMV up-to-date and small fraction of a base table is modified infrequently. Due to the overhead of immediate maintenance, IVM is not effective when a base table is modified frequently. Also, when a large part of a base table is modified or large data is inserted into a base table, IVM is not effective and the cost of maintenance can be larger than refresh from scratch.
In such situation, we can use `refesh_immv` function with `with_data = falase` to disable immediate maintenance before modifying a base table. After a base table modification, call `refresh_immv`with `with_data = true` to refresh the view data and enable immediate maintenance.
## Authors
IVM Development Group
- https://github.com/yugo-n