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From "John Phillips (JIRA)" <>
Subject [jira] [Commented] (PHOENIX-5239) Send persistent subquery cache to all regionservers
Date Thu, 02 May 2019 17:09:00 GMT


John Phillips commented on PHOENIX-5239:

[~lhofhansl] Thanks for getting back. I'm definitely open to updating the code to add a flag,
but based on your reply, I just want to first make sure we're on the same page as I realized
I may not have done a great job of explaining certain aspects of this change.

bq. we need to consider the general case
The PR as written does not change the behavior of the "general case". This _only_ applies
to {{SELECT}} queries where the "{{/*+ USE_PERSISTENT_CACHE */}}" hint is explicitly included.
Without that hint, the behavior of subqueries would be unchanged (they will only be distributed
to the regionservers that are strictly required for the query).

bq. We cannot default to shipping to shipping a subquery to 1000 or more server only because
they may be later be involved in a query
The use case of the {{USE_PERSISTENT_CACHE}} hint is when people plan on using the subquery
in later queries and can tolerate stale data. If someone is using the hint for a bunch of
one-off queries, that's a usage mistake on their end which I'm not convinced we necessarily
need to do anything to prevent.  Someone could just as easily write the outer query join condition
on some non-leading part of the primary key, which would have the same effect of distributing
the subquery to all the regionservers.

Last, the caching is done inside of tenant specific caches ({{TenantCacheImpl}}), which should
help isolate some of the memory impact in a multi-tenant environment.

But if all that makes sense, and you still feel there should be a flag for this, I'll go ahead
and update the PR.

> Send persistent subquery cache to all regionservers
> ---------------------------------------------------
>                 Key: PHOENIX-5239
>                 URL:
>             Project: Phoenix
>          Issue Type: Improvement
>            Reporter: John Phillips
>            Priority: Major
>          Time Spent: 0.5h
>  Remaining Estimate: 0h
> PHOENIX-4666 introduced a persistent subquery cache that allowed phoenix to cache the
results from an expensive subquery (enabled with a {{USE_PERSISTENT_CACHE}} query hint) to
speed up subsequent queries.
> More context is available on the PHOENIX-4666 ticket, but a quick example would be a
query like:
> {code:java}
>     FROM table1
>     JOIN (SELECT id_1 FROM large_table WHERE x = 10) expensive_result
>     ON table1.id_1 = expensive_result.id_2
> WHERE table1.id_1 = [some_id]
> {code}
> Where lots of queries are ran, differing only by {{some_id}}. Our usage involves first
running one query over phoenix to warm the cache (which takes ~20 seconds), then once complete,
allowing the live query to run which utilize the persistent subquery cache (~100ms).
> However, we noticed that when phoenix sends the cache to the regionservers, it looks
at {{some_id}} in the outer query to figure out which regionservers might contain {{table1.id_1
= [some_id]}} ([code here|]).
This means that when we first start running the query, we'll inconsistently hit the cache
until it ends up being propagated to all the regionservers.
> Basically, we'd like to have some way to warm the subquery cache and ensure it's on all
the regionservers so subsequent queries will always find the cache. I think the simplest solution
might be updating the [if statement in ServerCacheClient#addServerCache|]
to simply always send the cache to all the regionservers if it's a persistent subquery:
> {code:java}
> - if ( ! servers.contains(entry) &&
> -         keyRanges.intersectRegion(regionStartKey, regionEndKey,
> -                 cacheUsingTable.getIndexType() == IndexType.LOCAL)) {
> + boolean keyRangesIntersect = keyRanges.intersectRegion(regionStartKey, regionEndKey,
> +         cacheUsingTable.getIndexType() == IndexType.LOCAL);
> + if (!servers.contains(entry) && (keyRangesIntersect || usePersistentCache))
> {code}
> I tested this out, and it seems to work as expected. If it sounds like an acceptable
solution, I'd be happy to make an actual PR. Or, if anyone has any other suggestions on better
ways to handle this, it would be much appreciated.
> FYI [~jamestaylor], [~elserj], and [~maryannxue] since it looks like you three handled
most of the review on the [original persistent cache PR|]

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