This page is meant as a template for writing a FLIP. To create a FLIP choose Tools->Copy on this page and modify with your content and replace the heading with the next FLIP number and a description of your issue. Replace anything in italics with your own description.

Status

Current state: [One of "Under Discussion", "Accepted", "Rejected"]

Discussion thread: here (<- link to https://mail-archives.apache.org/mod_mbox/flink-dev/)

JIRA: here (<- link to https://issues.apache.org/jira/browse/FLINK-XXXX)

...

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

Describe the problems you are trying to solve.

Public Interfaces

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed. The purpose of this section is to concisely call out the public contract that will come along with this feature.

A public interface is any change to the following:

• Binary log format

• The network protocol and api behavior

• Any class in the public packages under clientsConfiguration, especially client configuration

  • org/apache/kafka/common/serialization

  • org/apache/kafka/common

  • org/apache/kafka/common/errors

  • org/apache/kafka/clients/producer

  • org/apache/kafka/clients/consumer (eventually, once stable)

• Monitoring

• Command line tools and arguments

• Anything else that will likely break existing users in some way when they upgrade

Proposed Changes

Often users need to adjust existing tables slightly. This is especially useful when users need to enhance a table created from an external tool (e.g. HIVE) with Flink's specific information such as e.g watermarks. It can also be a useful tool for ETL processes, e.g. merging two tables into a single one with a different connector.  My suggestion would be to support a variation of an optional Feature T171, “LIKE clause in table definition” of SQL standard 2008.

Proposed Changes

I suggest introducing a LIKE clause with a following syntax:

CREATE [ TEMPORARY ] TABLE [ IF NOT EXISTS ] table_name ( [
  {   column
    | table_constraint
    [, ... ]
] )
[ WITH ( table_properties ) ]
[ LIKE parent_table [ like_options ] } ]

where like_options are:

  { INCLUDING | EXCLUDING } ALL
| [{ 
     { INCLUDING | EXCLUDING } { CONSTRAINTS }
   | { INCLUDING | EXCLUDING | OVERWRITING } { GENERATED | OPTIONS } }
   [, ...]
  ]

Example:

CREATE [TEMPORARY] TABLE base_table (
    id BIGINT,
    name STRING,
    tstmp TIMESTAMP,
	PRIMARY KEY(id)
) WITH (
    ‘connector.type’: ‘kafka’
)

CREATE [TEMPORARY] TABLE derived_table (
    LIKE base_table,
    WATERMARK FOR tstmp AS tsmp - INTERVAL '5' SECOND
)
LIKE base_table;

Resulting table equivalent to:

CREATE [TEMPORARY] TABLE derived_table (
    id BIGINT,
    name STRING,
    tstmp TIMESTAMP,
	PRIMARY KEY(id),
	WATERMARK FOR tstmp AS tsmp - INTERVAL '5' SECOND
) WITH (
    ‘connector’: ‘kafka’
)

Configuring behaviour of LIKE:

SQL standard defines a way to configure the behaviour of LIKE clause. SQL standards describes options:

...

CONSTRAINTS | INDEXES | STORAGE | COMMENTS | ALL

I suggest supporting INCLUDING/EXCLUDINGlike-options:

• ALL
• CONSTRAINTS: primary keys, unique key, does not include NOT NULL constraint (in Flink it's part of the type)
  
• GENERATED: computed columns and watermarks
  
• OPTIONS: connector properties in WITH (...) clause
  
• PARTITIONS: partitions definition

Additionally I suggest supporting different INCLUDING/EXCLUDING like-options strategies:For the 

• INCLUDING OPTIONS (default -> fail on duplicate keys)
• EXCLUDING OPTIONS
• OVERWRITING OPTIONS (any explicit options/columns overwrite options/columns from base source, overwriting happens in the order the LIKE clauses were declared))
  

The supported combinations would be:

	INCLUDING	EXCLUDING	OVERWRITING
ALL
CONSTRAINTS
PARTITIONS
GENERATED
OPTIONS

The reason why I suggest not to support OVERWRITING CONSTRAINTS is that usually it's rather hard to overwrite only some of the constraints. If the constraint(primary key, unique key) from the base table does not apply to the derived table, in my opinion it is safer to exclude all constraints and redefine them.  We could revisit that in the future, if we see a need to support also OVERWRITING for constraints. The same applies for PARTITIONS.

Default options:

    INCLUDING ALL = INCLUDING CONSTRAINTS

                                                   INCLUDING GENERATED

                                       INCLUDING PARTITIONS

                                       INCLUDING OPTIONS

It is different from the SQL standard. Nevertheless I think it is worth the change as the primary use case for the clause will be slight adjustments to the original schema rather than rewriting the table completely.

...

CREATE [TEMPORARY] TABLE base_table_1 (
    id BIGINT,
    name STRING,
    tstmp TIMESTAMP,
	PRIMARY KEY(id)
) WITH (
    ‘connector’: ‘kafka’,
    ‘connector.starting-offset’: ‘12345’,
    ‘format’: ‘json’
)

CREATE [TEMPORARY] TABLE base_table_2 (
    tstmp TIMESTAMP,
	PRIMARY KEY(tstmp)
) WITH (
    ‘connector’: ‘filesystem’,
    ‘format’: ‘csv’,
    ‘format.delimiter’: ‘\t’   
)

CREATE [TEMPORARY] TABLE derived_table (
    LIKE base_table_1 (
		OVERWRITING SOURCE_OPTIONS),
    LIKE base_table_2 (
		EXCLUDING SOURCE_OPTIONS, 
		EXCLUDING CONSTRAINTS),
    WATERMARK FOR tstmp AS tsmp - INTERVAL '5' SECOND
) 
WITH (
    ‘connector.starting-offset’: ‘0’
)
LIKE base_table (OVERWRITING OPTIONS, EXCLUDING CONSTRAINTS)

The derived_table will be equivalent to:

CREATE [TEMPORARY] TABLE derived_table (
    id BIGINT,
    name STRING,
	    tstmp TIMESTAMP,
    WATERMARK FOR tstmp AS tsmp - INTERVAL '5' SECOND,
    PRIMARY KEY(id)
) WITH (
    ‘connector’: ‘kafka’,
    ‘connector.starting-offset’: ‘0’,
    ‘format’: ‘json’
)

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Test Plan

Describe in few sentences how the FLIP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?

Rejected Alternatives

Merging columns of the original table

The columns of the original table shall be appended to the columns defined in the current statement. If the are columns with the same name in both tables an exception will be thrown.

Support in Table API

Support of that feature in Table API will require a separate FLIP, as the connect API requires a rework anyway.

Compatibility, Deprecation, and Migration Plan

It is a new feature with no implication for backwards compatibility.

Rejected Alternatives

Original suggestion was to put the LIKE clause in the schema part. During the discussion it was mentioned that it is a bit weird that a clause in the schema part affects options in the WITH ( /* connector properties */) clause. We moved the LIKE clause out of the schema part.

Originally the suggestion was to allow multiple LIKE clauses. It was suggested to drop that for the first version and support it, if such requirement shows upIf there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.