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Executing statements in Clickhouse

Warning

Methods below read all the rows returned from DB to Spark driver memory, and then convert them to DataFrame.

Do NOT use them to read large amounts of data. Use DBReader or Clickhouse.sql instead.

How to

There are 2 ways to execute some statement in Clickhouse

Use Clickhouse.fetch

Use this method to perform some SELECT query which returns small number or rows, like reading Clickhouse config, or reading data from some reference table. Method returns Spark DataFrame.

Method accepts Clickhouse.FetchOptions.

Warning

Please take into account Clickhouse types.

Syntax support in Clickhouse.fetch

This method supports any query syntax supported by Clickhouse, like:

  • ✅︎ SELECT ... FROM ...
  • ✅︎ WITH alias AS (...) SELECT ...
  • ✅︎ SELECT func(arg1, arg2) - call function
  • ✅︎ SHOW ...
  • ❌ SET ...; SELECT ...; - multiple statements not supported

Examples for Clickhouse.fetch

from onetl.connection import Clickhouse

clickhouse = Clickhouse(...)

df = clickhouse.fetch(
    "SELECT value FROM some.reference_table WHERE key = 'some_constant'",
    options=Clickhouse.FetchOptions(queryTimeout=10),
)
clickhouse.close()
value = df.collect()[0][0]  # get value from first row and first column

Use Clickhouse.execute

Use this method to execute DDL and DML operations. Each method call runs operation in a separated transaction, and then commits it.

Method accepts Clickhouse.ExecuteOptions.

Syntax support in Clickhouse.execute

This method supports any query syntax supported by Clickhouse, like:

  • ✅︎ CREATE TABLE ..., CREATE VIEW ..., and so on
  • ✅︎ ALTER ...
  • ✅︎ INSERT INTO ... SELECT ..., UPDATE ..., DELETE ..., and so on
  • ✅︎ DROP TABLE ..., DROP VIEW ..., TRUNCATE TABLE, and so on
  • ✅︎ other statements not mentioned here
  • ❌ SET ...; SELECT ...; - multiple statements not supported

Examples for Clickhouse.execute

from onetl.connection import Clickhouse

clickhouse = Clickhouse(...)

clickhouse.execute("DROP TABLE schema.table")
clickhouse.execute(
    """
    CREATE TABLE schema.table (
        id UInt8,
        key String,
        value Float32
    )
    ENGINE = MergeTree()
    ORDER BY id
    """,
    options=Clickhouse.ExecuteOptions(queryTimeout=10),
)

Notes

These methods read all the rows returned from DB to Spark driver memory, and then convert them to DataFrame.

So it should NOT be used to read large amounts of data. Use DBReader or Clickhouse.sql instead.

Options

ClickhouseFetchOptions

Bases: JDBCFetchOptions

fetchsize = None class-attribute instance-attribute

How many rows to fetch per round trip.

Tuning this option can influence performance of reading.

Warning

Default value depends on driver. For example, Oracle has default fetchsize=10.

query_timeout = Field(default=None, alias='queryTimeout') class-attribute instance-attribute

The number of seconds the driver will wait for a statement to execute. Zero means there is no limit.

This option depends on driver implementation, some drivers can check the timeout of each query instead of an entire JDBC batch.

parse(options) classmethod

If a parameter inherited from the ReadOptions class was passed, then it will be returned unchanged. If a Dict object was passed it will be converted to ReadOptions.

Otherwise, an exception will be raised

ClickhouseExecuteOptions

Bases: JDBCExecuteOptions

fetchsize = None class-attribute instance-attribute

How many rows to fetch per round trip.

Tuning this option can influence performance of reading.

Warning

Default value depends on driver. For example, Oracle has default fetchsize=10.

query_timeout = Field(default=None, alias='queryTimeout') class-attribute instance-attribute

The number of seconds the driver will wait for a statement to execute. Zero means there is no limit.

This option depends on driver implementation, some drivers can check the timeout of each query instead of an entire JDBC batch.

parse(options) classmethod

If a parameter inherited from the ReadOptions class was passed, then it will be returned unchanged. If a Dict object was passed it will be converted to ReadOptions.

Otherwise, an exception will be raised