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https://issues.apache.org/jira/browse/HIVE-20623?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16626135#comment-16626135
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Jesus Camacho Rodriguez commented on HIVE-20623:
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Thanks [~gopalv], uploaded a new patch addressing the issue.
> Shared work: Extend sharing of map-join cache entries in LLAP
> -------------------------------------------------------------
>
> Key: HIVE-20623
> URL: https://issues.apache.org/jira/browse/HIVE-20623
> Project: Hive
> Issue Type: Improvement
> Components: llap, Logical Optimizer
> Reporter: Gopal V
> Assignee: Jesus Camacho Rodriguez
> Priority: Major
> Attachments: HIVE-20623.01.patch, HIVE-20623.02.patch,
> HIVE-20623.patch, hash-shared-work.json.txt, hash-shared-work.svg
>
>
> For a query like this
> {code}
> with all_sales as (
> select ss_customer_sk as customer_sk, ss_ext_list_price-ss_ext_discount_amt
> as ext_price from store_sales
> UNION ALL
> select ws_bill_customer_sk as customer_sk,
> ws_ext_list_price-ws_ext_discount_amt as ext_price from web_sales
> UNION ALL
> select cs_bill_customer_sk as customer_sk, cs_ext_sales_price -
> cs_ext_discount_amt as ext_price from catalog_sales)
> select sum(ext_price) total_price, c_customer_id from all_sales, customer
> where customer_sk = c_customer_sk
> group by c_customer_id
> order by total_price desc
> limit 100;
> {code}
> The hashtable used for all 3 joins are identical, which is loaded 3x times in
> the same LLAP instance because they are named.
> {code}
> cacheKey = "HASH_MAP_" + this.getOperatorId() + "_container";
> {code}
> in the cache.
> If those are identical in nature (i.e vectorization, hashtable type etc),
> then the duplication is just wasted CPU, memory and network - using the cache
> name for hashtables which will be identical in layout would be extremely
> useful.
> In cases where the join is pushed through a UNION, those are identical.
> This optimization can only be done without concern for accidental delays when
> the same upstream task is generating all of these hashtables, which is what
> is achieved by the shared scan optimizer already.
> In case the shared work is not present, this has potential downsides - in
> case two customer broadcasts were sourced from "Map 1" and "Map 2", the Map 1
> builder will block the other task from reading from Map 2, even though Map 2
> might have started after, but finished ahead of Map 1.
> So this specific optimization can always be considered for cases where the
> shared work unifies the operator tree and the parents of all the RS entries
> involved are same (& the RS layout is the same).
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