EmbeddedRocksDB Engine 

This engine allows integrating ClickHouse with rocksdb.

Creating a Table 

CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
    name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1],
    name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2],
    ...
) ENGINE = EmbeddedRocksDB PRIMARY KEY(primary_key_name)

Required parameters:

  • primary_key_name – any column name in the column list.
  • primary key must be specified, it supports only one column in the primary key. The primary key will be serialized in binary as a rocksdb key.
  • columns other than the primary key will be serialized in binary as rocksdb value in corresponding order.
  • queries with key equals or in filtering will be optimized to multi keys lookup from rocksdb.

Example:

CREATE TABLE test
(
    `key` String,
    `v1` UInt32,
    `v2` String,
    `v3` Float32,
)
ENGINE = EmbeddedRocksDB
PRIMARY KEY key

Metrics 

There is also system.rocksdb table, that expose rocksdb statistics:

SELECT
    name,
    value
FROM system.rocksdb

┌─name──────────────────────┬─value─┐
 no.file.opens                  1 
 number.block.decompressed      1 
└───────────────────────────┴───────┘

Configuration 

You can also change any rocksdb options using config:

<rocksdb>
    <options>
        <max_background_jobs>8</max_background_jobs>
    </options>
    <column_family_options>
        <num_levels>2</num_levels>
    </column_family_options>
    <tables>
        <table>
            <name>TABLE</name>
            <options>
                <max_background_jobs>8</max_background_jobs>
            </options>
            <column_family_options>
                <num_levels>2</num_levels>
            </column_family_options>
        </table>
    </tables>
</rocksdb>

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