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  1. MariaDB ColumnStore
  2. MCOL-4569

Queries with UNION ALL perform disproportionally badly

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    Description

      It gets much worse if a UNION ALL is involved.
      If you do
      select count(c) from (select * from t1) q;
      select count(c) from (select * from t2) q;
      and compare it to
      select count(c) from (select * from t1 union all select * from t2) q; the difference in elapsed and CPU is around 10x (second query is 10x worse than the sum of the first two).
      Again, plans and I/O stats are identical (second is the same as sum of the first two).

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          duplicates

          Task - A task that needs to be done. MCOL-4901 Allow pushdown of queries involving UNIONs in outer select to ColumnStore

          • Blocker - Blocks development and/or testing work, production could not run.
          • Closed
          is caused by

          Task - A task that needs to be done. MDEV-25080 Allow pushdown of queries involving UNIONs in outer select to foreign engines

          • Critical - Crashes, loss of data, severe memory leak.
          • Closed
          is duplicated by

          Task - A task that needs to be done. MCOL-4563 Investigate and correct a CPU / elapsed anomaly with UNION

          • Major - Major loss of function.
          • Closed
          relates to

          Task - A task that needs to be done. MCOL-4584 Significant performance degradation when UNION ALL is used in an outer select, compared to in a subquery.

          • Major - Major loss of function.
          • Closed
          split from

          Bug - A problem which impairs or prevents the functions of the product. MCOL-4543 Locate and Remove CPU waste in PM1 on nested queries with aggregates

          • Critical - Crashes, loss of data, severe memory leak.
          • Closed

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              Unassigned Unassigned
              gdorman Gregory Dorman (Inactive)
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                Updated:
                Resolved:

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