[jira] [Commented] (PHOENIX-5239) Send persistent subquery cache to all regionservers

["Lars Hofhansl (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/PHOENIX-5239?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16816683#comment-16816683 ] 

Lars Hofhansl commented on PHOENIX-5239:
----------------------------------------

What if there are 1000 regionserver, and your query only touches 10 of them?

> 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
>
> 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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