Makes we wonder under which circumstances an OAuth token would need to be looked up using LIKE. I would expect the unique b-tree index which covers exact lookups to be sufficient.
Simon Le mardi 14 avril 2020 09:01:48 UTC-4, Adam Johnson a écrit : > > For some numbers: I just checked on a long-running client project using > PostgreSQL, and it looks like several of the *_like indexes are in use: > > core=> select right(indexrelname, 10), idx_tup_read, idx_tup_fetch from > pg_stat_all_indexes where indexrelname like '%_like' order by 1; > right | idx_tup_read | idx_tup_fetch > ------------+--------------+--------------- > 090f8_like | 168181 | 168143 > 0b77e_like | 0 | 0 > 0cc84_like | 0 | 0 > 1ab7c_like | 6260 | 6137 > 32a5e_like | 2443202 | 2412585 > 33678_like | 0 | 0 > 37b91_like | 0 | 0 > 6c02a_like | 15 | 15 > 75088_like | 56746 | 9640 > 90e0f_like | 8371 | 8089 > a08ec_like | 2815 | 2815 > b4ddf_like | 0 | 0 > (12 rows) > > The first highest read index is accidental and the query will be rewritten. > The second-highest read index here though is from the third party app > django-oauth-toolkit: > https://github.com/jazzband/django-oauth-toolkit/blob/master/oauth2_provider/models.py#L284 > > . > The other indexes are on small tables and I'm not concerned about them > from a performance perspective. > > > Perhaps it's possible to use these index statistics in the system check to > prioritize which indexes are warned about. > > On Tue, 14 Apr 2020 at 13:18, Hannes Ljungberg <hannes....@gmail.com > <javascript:>> wrote: > >> Thanks for your reply Tim, >> >> I also fail to see a "clean" upgrade path. The way I’ve been thinking of, >> is to just as you wrote, keep the code for deletion of the index so people >> who migrate to the Index-class will get it removed properly. Since it uses >> `IF EXISTS` it should be future-proof. We could add a system check that >> uses introspection to check if the `*_like` index is present on affected >> fields with `db_index=True` and warn about its deprecation. Then after a >> couple (?) of versions we could remove all traces of this index. >> >> The negative performance impact of removing this index for users of >> `LIKE` queries is very large but it’s hard to give numbers since it depends >> on the amount and nature of the data in the affected rows. Since we >> wouldn’t do any automatic removal of the index and add documentation about >> the importance of the opclass-indexes for `LIKE` queries I’m not that >> worried. >> >> I realise that removing the creation of these indexes result in a not so >> pretty deprecation path but I think users of Django would benefit from >> having to be more explicit about their indexes and learn about the quirks >> of their database engines. >> >> >> Den tisdag 14 april 2020 kl. 03:01:03 UTC+2 skrev Tim Graham: >>> >>> I have some sympathy for this issue as I'm trying to make the >>> createcachetable management command use SchemaEditor rather than some >>> custom SQL construction logic*. The related problem I ran into is that the >>> primary key column (a CharField) uses unique=True which means those >>> undesired opclasses indexes are created. I couldn't find a way to >>> prevent that index besides filtering it out of the list of SQL statements. >>> >>> As for your proposal, how would you handle the upgrade path for existing >>> projects? I imagine we could provide a script to run upon upgrade to remove >>> all such existing indexes. No doubt some users won't run it. Would you keep >>> around the code in Django's SchemaEditor like >>> https://github.com/django/django/blob/53d229ff632c2a3e547f2820a94239f38ba4d4ac/django/db/backends/postgresql/schema.py#L178-L180 >>> >>> that assumes those indexes exist? >>> >>> If Django stops creating those indexes, it could create a somewhat murky >>> situation for some developers as they try to figure out the state of >>> indexes in their database. It might depend on which version of Django was >>> in use when certain migrations ran. >>> >>> Some developers might be left debugging performance issues if those >>> indexes are removed. It could be helpful to gives some numbers as to the >>> possible performance impact on applications if these indexes are removed >>> without a developer realizing it. >>> >>> Third-party apps that need it could add an Index with opclasses but >>> then they'd face the issue of duplicate opclasses indexes if their app is >>> used on an older version of Django. >>> >>> * https://github.com/django/django/pull/12590 >>> >>> On Sunday, April 12, 2020 at 10:30:50 AM UTC-4, Hannes Ljungberg wrote: >>>> >>>> Hi all, >>>> >>>> I would like to continue the discussion started in this very old thread: >>>> >>>> https://groups.google.com/d/msg/django-developers/H2QFcQYsbo8/RmRb-8FVypwJ >>>> >>>> I’m sorry if I should've continued the discussion in that thread but it >>>> felt a bit wrong to bring a 5 year old thread back to life :-) >>>> >>>> Anyway, as previously described in that thread the implicit creation of >>>> the `*_pattern_ops` index when creating indexes on `CharField` and >>>> `TextField` with `db_index=True` is not ideal. >>>> >>>> In my experience `LIKE` expressions is not that common that it warrants >>>> to always create an index to cover this. >>>> >>>> For very large tables this can become a problem where insertion/update >>>> performance is negatively affected and disc space usage is twice what >>>> really is needed if no `LIKE` queries are used. >>>> >>>> And even if one would like to use `LIKE` expressions it’s not obvious >>>> that the `*_pattern_ops` is the correct index. For leading wildcard >>>> patters >>>> as `LIKE %foo` one has to use a GIN/GiST index with the `*_trgm_ops` >>>> opclass. With the current implementation we would end up with 3 indexes >>>> when 2 would be sufficient for this use case (the regular b-tree and the >>>> trigram). >>>> >>>> One could also argue that we’re not consistent with these implicit >>>> indexes. The `iexact`/`icontains` lookups require an expression index on >>>> `(UPPER(col))` but that’s not created. >>>> >>>> One important detail is that this implicit index is _not_ created when >>>> using the class based `Index` . In my opinion it’s not very clear that one >>>> needs to handle the creation of a `*_pattern_ops` index manually when >>>> using >>>> it. >>>> >>>> This is the only documentation that I’ve been able to find about the >>>> creation of these implicit indexes: >>>> https://docs.djangoproject.com/en/dev/ref/databases/#indexes-for-varchar-and-text-columns >>>> >>>> My proposal is to remove the implicit index created by `db_index=True` >>>> and add documentation that one should use `Index.opclasses` to utilise >>>> indexes for `LIKE` queries on PostgresSQL. This would give a more >>>> consistent behaviour and being more explicit about this would help people >>>> tune their indexes. >>>> >>>> If there’s a consensus on this I would like to give this a shot. If >>>> there’s not an agreement on removing the implicit index I think we should >>>> at least make the documentation around this a bit more clear about the >>>> gotchas of `Index` vs `db_index=True`. >>>> >>>> Here’s two tickets which have previously discussed this: >>>> https://code.djangoproject.com/ticket/24507 >>>> https://code.djangoproject.com/ticket/24088 >>>> >>>> Stay safe! >>>> >>>> -- >> You received this message because you are subscribed to the Google Groups >> "Django developers (Contributions to Django itself)" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to django-d...@googlegroups.com <javascript:>. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/django-developers/6b539bf5-4de2-434f-b022-8729bf49a593%40googlegroups.com >> >> <https://groups.google.com/d/msgid/django-developers/6b539bf5-4de2-434f-b022-8729bf49a593%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > > > -- > Adam > -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. 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