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netzkolchose / django-fast-update

Faster db updates using UPDATE FROM VALUES sql variants.
MIT License
20 stars 2 forks source link

expression support #10

Closed jerch closed 2 years ago

jerch commented 2 years ago

To get feature parity, fast_update should get support for expression values.

We might get away by simply prefiltering objects with expression values and update them with bulk_update beforehand. This should be equivalent for dbs with standard UPDATE implementation, which normally does not allow window functions directly. This needs some serious tests against mysql (non-standard UPDATE impl), and CTE/subquery windowing tricks (updatable views - not sure if those can be expressed by django ORM at all).

Plan B would be to apply the same CASE chain idea to the SET clause as bulk_update but only for fields with expression values, The chains also would be much shorter, given that most values are literal values from the values table. Yet the creation of those chains is cumbersome, also the runtime will suffer again, so thats only the second best option.

jerch commented 2 years ago

Obsolete now, as we cannot solve it without a reliable batching/duplicates plan. Resorting expression updates would always lead to update edge cases not properly covered without strict duplicate handling. (Linked to #11).