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Executing statements in Postgres

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 Postgres.sql instead.

How to

There are 2 ways to execute some statement in Postgres

Use Postgres.fetch

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

Method accepts Postgres.FetchOptions.

Warning

Please take into account Postgres types.

Syntax support in Postgres.fetch

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

  • ✅︎ SELECT ... FROM ...
  • ✅︎ WITH alias AS (...) SELECT ...
  • ❌ SET ...; SELECT ...; - multiple statements not supported

Examples for Postgres.fetch

from onetl.connection import Postgres

postgres = Postgres(...)

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

Use Postgres.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 Postgres.ExecuteOptions.

Syntax support in Postgres.execute

This method supports any query syntax supported by Postgres, 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
  • ✅︎ CALL procedure(arg1, arg2) ...
  • ✅︎ SELECT func(arg1, arg2) or {call func(arg1, arg2)} - special syntax for calling functions
  • ✅︎ other statements not mentioned here
  • ❌ SET ...; SELECT ...; - multiple statements not supported

Examples for Postgres.execute

from onetl.connection import Postgres

postgres = Postgres(...)

postgres.execute("DROP TABLE schema.table")
postgres.execute(
    """
    CREATE TABLE schema.table (
        id bigint GENERATED ALWAYS AS IDENTITY,
        key text,
        value real
    )
    """,
    options=Postgres.ExecuteOptions(queryTimeout=10),
)

Options

PostgresFetchOptions

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

PostgresExecuteOptions

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