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  1. MariaDB Server
  2. MDEV-24563

variable federated_pushdown is not documented and buggy

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      The variable federated_pushdown is not well documented:

      https://mariadb.com/kb/en/about-federatedx/
      https://mariadb.com/kb/en/server-system-variables/
      http://lists.askmonty.org/pipermail/commits/2019-February/013402.html
      https://jira.mariadb.org/browse/MDEV-23803

      The only information found was this:

      storage/federatedx/federatedx_pushdown.cc:

      /*
      This is a quick a dirty implemention of the derived_handler and select_handler
      interfaces to be used to push select queries and the queries specifying
      derived tables into FEDERATEDX engine.
      The functions
      create_federatedx_derived_handler and
      create_federatedx_select_handler
      that return the corresponding interfaces for pushdown capabilities do
      not check a lot of things. In particular they do not check that the tables
      of the pushed queries belong to the same foreign server.

      The implementation is provided purely for testing purposes.
      The pushdown capabilities are enabled by turning on the plugin system
      variable federated_pushdown:
      set global federated_pushdown=1;
      */

      It looks like it works like this:

      set global federated_pushdown=off;
      select id, data from test_fed where id = 1;
      -> Query on remote system: SELECT `id`, `data`, `ts` FROM `test` WHERE (`id` = 1)

      set global federated_pushdown=on;
      select id, data from test_fed where id = 1;
      -> Query on remote system: select id, data from test_fed where id = 1

      But this miserably failed with the following error (which is the/a bug):

      ERROR 1030 (HY000): Got error 10000 "Unknown error 10000" from storage engine FEDERATED

      Would be a cool feature to reduce traffic between the 2 systems, if it worked.

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            Assignee:
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            Reporter:
            oli Oli Sennhauser
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              Updated:

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