[jira] [Created] (IGNITE-6202) SQL: support hash join

["Vladimir Ozerov (JIRA)" <ji...@apache.org>]

Vladimir Ozerov created IGNITE-6202:
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             Summary: SQL: support hash join
                 Key: IGNITE-6202
                 URL: https://issues.apache.org/jira/browse/IGNITE-6202
             Project: Ignite
          Issue Type: Task
          Components: sql
    Affects Versions: 2.1
            Reporter: Vladimir Ozerov


Justification is the same as for IGNITE-6201 (merge joins). If join is equijoin, and participating attributes are not sorted in advance, then instead of executing nested loops, we can do the following:
1) Get the table with smaller number of entries
2) Build a hash table from target attribute to the list of matching entries
3) Iterate over larger table and perform lookup into hash table

Note that this operation might be memory-intensive in case smaller table is large enough still. For this reason we should provide a mechanism to avoid out-of-memory errors. Let's avoid spilling to disk in the first iteration, and try to set a kind of hard limit on how much data can be kept in memory. If we see that this limit cannot be satisfied, then fallback to nested loops.



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