Vladimir Ozerov created IGNITE-6089:
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Summary: SQL: Improve query parallelism architecture
Key: IGNITE-6089
URL: https://issues.apache.org/jira/browse/IGNITE-6089
Project: Ignite
Issue Type: Task
Components: sql
Affects Versions: 2.1
Reporter: Vladimir Ozerov
Currently query parallelism implement with static split of all indexes
(including PK) for cache. This approach has several major disadvantages:
1) It improves scans, but slows down index and range lookups
2) Tables with different DOP cannot be used in the same query
We need to fix that. Proposed plan:
1) No more index splits, ever - there is one and only one index always
2) Use preliminary execution plan, statistics (IGNITE-6079), CPU cores count
and CPU load to estimate whether query will benefit from parallelism.
3) if yes - split node-s single map query into several independent pieces.
Splitting can be achieved in one of the following ways:
1) Partition-based: e.g. if node owns partitions A, B, C and D, then we can
split it to two queries - one over (A, B), another over (C, D). This could be
useful for pure scans (e.g. DWH)
2) Histogram-based: e.g. if we have a query {{SELECT ... WHERE salary > 50}},
and we know salary distribution, we can split it into {{WHERE salary > 50 AND
salary <= 200}} and {{WHERE salary > 200}}
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