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From "Gourav Agrawal (JIRA)" <j...@apache.org>
Subject [jira] [Created] (IGNITE-10855) Delayed execution when running SQL query involving JOINS and ORDER BY eventually leading to heap OOME.
Date Wed, 02 Jan 2019 09:22:00 GMT
Gourav Agrawal created IGNITE-10855:
---------------------------------------

             Summary: Delayed execution when running SQL query involving JOINS and ORDER BY
eventually leading to heap OOME.
                 Key: IGNITE-10855
                 URL: https://issues.apache.org/jira/browse/IGNITE-10855
             Project: Ignite
          Issue Type: Bug
          Components: cache, sql
    Affects Versions: 2.7
            Reporter: Gourav Agrawal
             Fix For: None


To simplify our use-case, we created two caches using the SQL query and loaded data consisting
of about 4 million records and 60k records approximately, in the respective caches with INDEX
created on all the columns. Ignite is set up to run on a single node, meaning all the data
is present on the same node. The query used for testing/the one we are facing issue with is
of the type -

_SELECT * FROM CACHE1 C1, CACHE2 C2  WHERE  C1.JOINCol = C2.JOINCol AND C1.COL1 = 'someValue'
ORDER BY C1.COL2_

The above query execution leads to the Ignite thread memory rising extensively, eventually
leading to heap OOME. When the heap memory was increased to about 14GB,  we were able to
get the results back, but the processing time of the query was too long, about 2-4 minutes
( with CPUs =2).

We ran an EXPLAIN for the above query and found out that INDEX was created on COL1 for C1
cache and on JOINCol for C2 cache. There was no index on the sorted column. We think the problem
of 'slow querying and huge heap memory requirement' is because of the absence of an index
in the sorted column. Whenever there is a condition present in the WHERE clause ( in our example
C1.COL1='someValue'), Ignite is using an INDEX for that column and there is no INDEX being
created on the ORDER BY column.

And for our use-case, it is imperative that we have a condition in the where clause ( to filter
out the data) and a join condition apart from the order by clause.

 We tried the multiple column indexing strategy on the COL1, COL2 as per our use case.

 In case of a composite index with the order as (COL1, COL2), INDEX was created only for
the COL1.

While for the composite index order as (COL2, COL1), INDEX was getting created for both COL1
and COL2 and the results were index sorted. ( But only in case of the absence of an INDEX
for COL1, it looks for the ORDER BY clause column and uses a composite index). But, if we
don't have a separate INDEX for COL1, it again poses a problem as COL1 is something which
is heavily used for filtering in all other queries. So an INDEX on COL1 is necessary.

To summarize, In case there is a condition present in the WHERE clause, Ignite uses the WHERE
clause column for indexing, and therefore there is no INDEX in the sorting column, resulting
in severe query performance, which can eventually lead us to our system going down.



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