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From "Josh Elser (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (PHOENIX-5239) Send persistent subquery cache to all regionservers
Date Thu, 02 May 2019 18:26:00 GMT

    [ https://issues.apache.org/jira/browse/PHOENIX-5239?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16831840#comment-16831840
] 

Josh Elser commented on PHOENIX-5239:
-------------------------------------

Late to the party (still working off emails after PTO). I share Lars' concern – while this
approach would work well if you're the sole consumer of a cluster or you have a relatively
small cluster (say <20 nodes), I think it really breaks down at scale.
{quote}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.
{quote}
 
{quote} this would just be for persistently cached subqueries and regular subqueries would
continue to send the cache to only the 10 relevant servers for the query.
{quote}
 
{quote}if following queries occurred serially (same example query, just substituting out the
where clause in the outer query), I believe the current code would produce this behavior:
{quote}
It seems to me that the problem is that, upon re-use of some already-computed persistent query,
the implementation doesn't ensure that it's on the RS's that you need for the new query.

Take two queries: Q1 and Q2. Q1 and Q2 differ only by the where clause as you describe above.
They both reference some same inner query whose results are cache in some persistent query
(PQ1). Say Q1 involves a subset of RegionServers RS1 and Q2 involves a disjoint subset RS2.

The problem is that after running Q1, RS1 has PQ1 cached, but RS2 does not. Thus, when running
Q2, the query fails/errors/is-slow because you have to cache PQ1 on RS2. Is this correct?
Why, when Q2 starts, wouldn't it make sure that it's cached on RS2?

I do recall that persistent queries are using a separate ServerCache (which helps avoid thrashing
cache concerns), but I would still be concerned about this implementation scaling up as you
get to 10, 20, 50, 100 RegionServers. You're going to be paying a linear cost (at best) for
persistent queries WRT your RegionServers which isn't good.

> Send persistent subquery cache to all regionservers
> ---------------------------------------------------
>
>                 Key: PHOENIX-5239
>                 URL: https://issues.apache.org/jira/browse/PHOENIX-5239
>             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}
> SELECT /*+ USE_PERSISTENT_CACHE */ *
>     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|https://github.com/apache/phoenix/blob/2084a6c/phoenix-core/src/main/java/org/apache/phoenix/cache/ServerCacheClient.java#L282-L283]).
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|https://github.com/apache/phoenix/blob/2084a6c/phoenix-core/src/main/java/org/apache/phoenix/cache/ServerCacheClient.java#L282-L283]
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|https://github.com/apache/phoenix/pull/298]



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