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223 lines
6.0 KiB
Markdown
223 lines
6.0 KiB
Markdown
---
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slug: /en/engines/table-engines/integrations/embedded-rocksdb
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sidebar_position: 50
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sidebar_label: EmbeddedRocksDB
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---
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# EmbeddedRocksDB Engine
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This engine allows integrating ClickHouse with [rocksdb](http://rocksdb.org/).
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## Creating a Table {#creating-a-table}
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``` sql
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CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
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(
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name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1],
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name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2],
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...
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) ENGINE = EmbeddedRocksDB([ttl, rocksdb_dir, read_only]) PRIMARY KEY(primary_key_name)
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[ SETTINGS name=value, ... ]
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```
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Engine parameters:
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- `ttl` - time to live for values. TTL is accepted in seconds. If TTL is 0, regular RocksDB instance is used (without TTL).
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- `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`.
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- `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.
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- `primary_key_name` – any column name in the column list.
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- `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`.
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- columns other than the primary key will be serialized in binary as `rocksdb` value in corresponding order.
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- queries with key `equals` or `in` filtering will be optimized to multi keys lookup from `rocksdb`.
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Engine settings:
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- `optimize_for_bulk_insert` – Table is optimized for bulk insertions (insert pipeline will create SST files and import to rocksdb database instead of writing to memtables); default value: `1`.
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- `bulk_insert_block_size` - Minimum size of SST files (in term of rows) created by bulk insertion; default value: `1048449`.
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Example:
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``` sql
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CREATE TABLE test
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(
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`key` String,
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`v1` UInt32,
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`v2` String,
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`v3` Float32
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)
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ENGINE = EmbeddedRocksDB
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PRIMARY KEY key
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```
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## Metrics
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There is also `system.rocksdb` table, that expose rocksdb statistics:
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```sql
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SELECT
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name,
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value
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FROM system.rocksdb
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┌─name──────────────────────┬─value─┐
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│ no.file.opens │ 1 │
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│ number.block.decompressed │ 1 │
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└───────────────────────────┴───────┘
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```
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## Configuration
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You can also change any [rocksdb options](https://github.com/facebook/rocksdb/wiki/Option-String-and-Option-Map) using config:
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```xml
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<rocksdb>
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<options>
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<max_background_jobs>8</max_background_jobs>
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</options>
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<column_family_options>
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<num_levels>2</num_levels>
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</column_family_options>
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<tables>
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<table>
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<name>TABLE</name>
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<options>
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<max_background_jobs>8</max_background_jobs>
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</options>
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<column_family_options>
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<num_levels>2</num_levels>
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</column_family_options>
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</table>
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</tables>
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</rocksdb>
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```
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By default trivial approximate count optimization is turned off, which might affect the performance `count()` queries. To enable this
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optimization set up `optimize_trivial_approximate_count_query = 1`. Also, this setting affects `system.tables` for EmbeddedRocksDB engine,
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turn on the settings to see approximate values for `total_rows` and `total_bytes`.
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## Supported operations {#supported-operations}
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### Inserts
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When new rows are inserted into `EmbeddedRocksDB`, if the key already exists, the value will be updated, otherwise a new key is created.
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Example:
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```sql
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INSERT INTO test VALUES ('some key', 1, 'value', 3.2);
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```
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### Deletes
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Rows can be deleted using `DELETE` query or `TRUNCATE`.
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```sql
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DELETE FROM test WHERE key LIKE 'some%' AND v1 > 1;
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```
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```sql
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ALTER TABLE test DELETE WHERE key LIKE 'some%' AND v1 > 1;
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```
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```sql
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TRUNCATE TABLE test;
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```
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### Updates
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Values can be updated using the `ALTER TABLE` query. The primary key cannot be updated.
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```sql
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ALTER TABLE test UPDATE v1 = v1 * 10 + 2 WHERE key LIKE 'some%' AND v3 > 3.1;
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```
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### Joins
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A special `direct` join with EmbeddedRocksDB tables is supported.
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This direct join avoids forming a hash table in memory and accesses
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the data directly from the EmbeddedRocksDB.
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With large joins you may see much lower memory usage with direct joins
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because the hash table is not created.
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To enable direct joins:
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```sql
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SET join_algorithm = 'direct, hash'
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```
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:::tip
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When the `join_algorithm` is set to `direct, hash`, direct joins will be used
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when possible, and hash otherwise.
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:::
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#### Example
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##### Create and populate an EmbeddedRocksDB table:
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```sql
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CREATE TABLE rdb
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(
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`key` UInt32,
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`value` Array(UInt32),
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`value2` String
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)
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ENGINE = EmbeddedRocksDB
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PRIMARY KEY key
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```
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```sql
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INSERT INTO rdb
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SELECT
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toUInt32(sipHash64(number) % 10) as key,
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[key, key+1] as value,
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('val2' || toString(key)) as value2
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FROM numbers_mt(10);
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```
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##### Create and populate a table to join with table `rdb`:
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```sql
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CREATE TABLE t2
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(
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`k` UInt16
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)
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ENGINE = TinyLog
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```
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```sql
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INSERT INTO t2 SELECT number AS k
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FROM numbers_mt(10)
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```
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##### Set the join algorithm to `direct`:
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```sql
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SET join_algorithm = 'direct'
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```
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##### An INNER JOIN:
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```sql
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SELECT *
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FROM
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(
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SELECT k AS key
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FROM t2
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) AS t2
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INNER JOIN rdb ON rdb.key = t2.key
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ORDER BY key ASC
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```
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```response
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┌─key─┬─rdb.key─┬─value──┬─value2─┐
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│ 0 │ 0 │ [0,1] │ val20 │
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│ 2 │ 2 │ [2,3] │ val22 │
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│ 3 │ 3 │ [3,4] │ val23 │
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│ 6 │ 6 │ [6,7] │ val26 │
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│ 7 │ 7 │ [7,8] │ val27 │
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│ 8 │ 8 │ [8,9] │ val28 │
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│ 9 │ 9 │ [9,10] │ val29 │
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└─────┴─────────┴────────┴────────┘
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```
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### More information on Joins
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- [`join_algorithm` setting](/docs/en/operations/settings/settings.md#join_algorithm)
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- [JOIN clause](/docs/en/sql-reference/statements/select/join.md)
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