Team Exasol
Team Exasol

Question

What is a smart table replication and how can the replication border be modified?

Answer

General information on smart table replication

  • Replicates are used "on-the-fly" for local joins if a table is "small" regarding the threshold.
  • A replicated table accesses data directly from other nodes' database memories and keeps a local copy in its own DBRAM.
  • If a replicated table is modified, only changed data is reloaded into database memories of other nodes.
  • Modified tables and subselects cannot be used with smart table replication. Table replication border does not apply to those.
  •  
    💡 Using large replicated tables might cause decreased performance. Queries with expensive table scans (filter expressions) or between / cross joins may fall into this category.

Soft replication border

A table will be replicated if none of the thresholds below are reached. The table size threshold refers to the RAW_OBJECT_SIZE like in EXA_*_OBJECT_SIZES.
The replication borders can be modified through extra database parameters in the web interface:

-soft_replicationborder_in_numrows=<numrows> [default is 100000 rows]
-soft_replicationborder_in_kb=<kb> [default is 1000000 -> 1GB]