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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([ttl, rocksdb_dir, read_only]) PRIMARY KEY(primary_key_name)

Engine parameters:

  • ttl - time to live for values. TTL is accepted in seconds. If TTL is 0, regular RocksDB instance is used (without TTL).
  • rocksdb_dir - path to the directory of an existed RocksDB or the destination path of the created RocksDB. Open the table with the specified rocksdb_dir.
  • read_only - when read_only is set to true, read-only mode is used. For storage with TTL, compaction will not be triggered (neither manual nor automatic), so no expired entries are removed.
  • 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>