Cloudera Docs

Schema design limitations

Kudu currently has some known limitations that may factor into schema design.

Primary key

  • The primary key cannot be changed after the table is created. You must drop and recreate a table to select a new primary key.

  • The columns which make up the primary key must be listed first in the schema.

  • The primary key of a row cannot be modified using the UPDATE functionality. To modify a row’s primary key, the row must be deleted and re-inserted with the modified key. Such a modification is non-atomic.

  • Columns with DOUBLE, FLOAT, or BOOL types are not allowed as part of a primary key definition. Additionally, all columns that are part of a primary key definition must be NOT NULL.

  • Auto-generated primary keys are not supported.

  • Cells making up a composite primary key are limited to a total of 16KB after internal composite-key encoding is done by Kudu.

Cells

No individual cell may be larger than 64KB before encoding or compression. The cells making up a composite key are limited to a total of 16KB after the internal composite-key encoding done by Kudu. Inserting rows not conforming to these limitations will result in errors being returned to the client.

Columns

  • By default, Kudu tables can have a maximum of 300 columns. We recommend schema designs that use fewer columns for best performance.

  • CHAR, DATE, and complex types such as ARRAY, MAP, and STRUCT are not supported.

  • Type, nullability, and type attributes (i.e. precision and scale of DECIMAL, length of VARCHAR) of the existing columns cannot be changed by altering the table.

  • Dropping a column does not immediately reclaim space. Compaction must run first.

Tables

  • Tables must have an odd number of replicas, with a maximum of 7.

  • Replication factor (set at table creation time) cannot be changed.

  • There is no way to run compaction manually, but dropping a table will reclaim the space immediately.

Other usage limitations

  • Secondary indexes are not supported.

  • Multi-row transactions are not supported.

  • Relational features, such as foreign keys, are not supported.

  • Identifiers such as column and table names are restricted to be valid UTF-8 strings. Additionally, a maximum length of 256 characters is enforced.

    If you are using Apache Impala to query Kudu tables, refer to the section on Impala integration limitations as well.