Create Table
This documentation is for an unreleased version of Apache Flink Table Store. We recommend you use the latest stable version.

Create Table #

Catalog #

Table Store uses its own catalog to manage all the databases and tables. Users need to configure the type table-store and a root directory warehouse to use it.

CREATE CATALOG my_catalog WITH (
  'type'='table-store',
  'warehouse'='hdfs://nn:8020/warehouse/path' -- or 'file://tmp/foo/bar'
);

USE CATALOG my_catalog;

Table Store catalog supports SQL DDL commands:

  • CREATE TABLE ... PARTITIONED BY
  • DROP TABLE ...
  • ALTER TABLE ...
  • SHOW DATABASES
  • SHOW TABLES

Syntax #

CREATE TABLE [IF NOT EXISTS] [catalog_name.][db_name.]table_name
  (
    { <physical_column_definition> | <computed_column_definition> }[ , ...n]
    [ <watermark_definition> ]
    [ <table_constraint> ][ , ...n]
  )
  [PARTITIONED BY (partition_column_name1, partition_column_name2, ...)]
  WITH (key1=val1, key2=val2, ...)
   
<physical_column_definition>:
  column_name column_type [ <column_constraint> ] [COMMENT column_comment]
  
<column_constraint>:
  [CONSTRAINT constraint_name] PRIMARY KEY NOT ENFORCED

<table_constraint>:
  [CONSTRAINT constraint_name] PRIMARY KEY (column_name, ...) NOT ENFORCED

<computed_column_definition>:
  column_name AS computed_column_expression [COMMENT column_comment]

<watermark_definition>:
  WATERMARK FOR rowtime_column_name AS watermark_strategy_expression

Note:

  • To ensure the uniqueness of the primary key, the primary key must contain the partition field.
  • Metadata column is not supported yet.

This will create a directory under ${warehouse}/${database_name}.db/${table_name}.

Table Options #

Important options include the following:

Option Required Default Type Description
bucket
Yes 1 Integer The bucket number for table store.
log.system
No (none) String The log system used to keep changes of the table, supports 'kafka'.
kafka.bootstrap.servers
No (none) String Required Kafka server connection string for log store.
kafka.topic
No (none) String Topic of this kafka table.

Distribution #

The data distribution of Table Store consists of three concepts: Partition, Bucket, and Primary Key.

CREATE TABLE MyTable (
  user_id BIGINT,
  item_id BIGINT,
  behavior STRING,
  dt STRING,
  PRIMARY KEY (dt, user_id) NOT ENFORCED
) PARTITIONED BY (dt) WITH (
  'bucket' = '4'
);

For example, the above MyTable has its data distribution in the following order:

  • Partition: isolating different data based on partition fields.
  • Bucket: Within a single partition, distributed into 4 different buckets based on the hash value of the primary key.
  • Primary key: Within a single bucket, sorted by primary key to build LSM structure.

Partition #

Table Store adopts the same partitioning concept as Apache Hive to separate data, and thus various operations can be managed by partition as a management unit.

Partitioned filtering is the most effective way to improve performance, your query statements should contain partition filtering conditions as much as possible.

Bucket #

Bucket is the concept of dividing data into more manageable parts for more efficient queries.

With N as bucket number, records are falling into (0, 1, ..., N-1) buckets. For each record, which bucket it belongs is computed by the hash value of one or more columns (denoted as bucket key), and mod by bucket number.

bucket_id = hash_func(bucket_key) % num_of_buckets

Users can specify the bucket key as follows

CREATE TABLE MyTable (
  catalog_id BIGINT,
  user_id BIGINT,
  item_id BIGINT,
  behavior STRING,
  dt STRING
) WITH (
    'bucket-key' = 'catalog_id'
);

Note:

  • If users do not specify the bucket key explicitly, the primary key (if present) or the whole row is used as bucket key.
  • Bucket key cannot be changed once the table is created. ALTER TALBE SET ('bucket-key' = ...) or ALTER TABLE RESET ('bucket-key') will throw exception.

The number of buckets is very important as it determines the worst-case maximum processing parallelism. But it should not be too big, otherwise, the system will create a lot of small files.

In general, the desired file size is 128 MB, the recommended data to be kept on disk in each sub-bucket is about 1 GB.

Primary Key #

The primary key is unique and indexed.

Flink Table Store imposes an ordering of data, which means the system will sort the primary key within each bucket. All fields will be used to sort if no primary key is defined. Using this feature, you can achieve high performance by adding filter conditions on the primary key.

The primary key’s choice is critical, especially when setting the composite primary key. A rule of thumb is to put the most frequently queried field in the front. For example:

CREATE TABLE MyTable (
  catalog_id BIGINT,
  user_id BIGINT,
  item_id BIGINT,
  behavior STRING,
  dt STRING,
  ......
);

For this table, assuming that the composite primary keys are the catalog_id and user_id fields, there are two ways to set the primary key:

  1. PRIMARY KEY (user_id, catalog_id)
  2. PRIMARY KEY (catalog_id, user_id)

The two methods do not behave in the same way when querying. Use approach one if you have a large number of filtered queries with only user_id, and use approach two if you have a large number of filtered queries with only catalog_id.

Partial Update #

You can configure partial update from options:

CREATE TABLE MyTable (
  product_id BIGINT,
  price DOUBLE,
  number BIGINT,
  detail STRING,
  PRIMARY KEY (product_id) NOT ENFORCED
) WITH (
  'merge-engine' = 'partial-update'
);

INSERT INTO MyTable
SELECT product_id, price, number, CAST(NULL AS STRING) FROM Src1 UNION ALL
SELECT product_id, CAST(NULL AS DOUBLE), CAST(NULL AS BIGINT), detail FROM Src2;

The value fields are updated to the latest data one by one under the same primary key, but null values are not overwritten.

For example, the inputs:

  • <1, 23.0, 10, NULL>
  • <1, NULL, NULL, ‘This is a book’>
  • <1, 25.2, NULL, NULL>

Output:

  • <1, 25.2, 10, ‘This is a book’>

Note:

  • Partial update is only supported for table with primary key.
  • Partial update is not supported for streaming consuming.
  • It is best not to have NULL values in the fields, NULL will not overwrite data.

Pre-aggregate #

You can configure pre-aggregate for each value field from options:

CREATE TABLE MyTable (
  product_id BIGINT,
  price DOUBLE,
  sales BIGINT,
  PRIMARY KEY (product_id) NOT ENFORCED
) WITH (
  'merge-engine' = 'aggregation',
  'fields.price.aggregate-function'='max',
  'fields.sales.aggregate-function'='sum'
);

Each value field is aggregated with the latest data one by one under the same primary key according to the aggregate function.

For example, the inputs:

  • <1, 23.0, 15>
  • <1, 30.2, 20>

Output:

  • <1, 30.2, 35>

Supported aggregate functions include sum, max/min, last_non_null_value, last_value, listagg, bool_or/bool_and. These functions support different data types.

  • sum supports DECIMAL, TINYINT, SMALLINT, INTEGER, BIGINT, FLOAT, DOUBLE data types.
  • max/min support DECIMAL, TINYINT, SMALLINT, INTEGER, BIGINT, FLOAT, DOUBLE, DATE, TIME, TIMESTAMP, TIMESTAMP_LTZ data types.
  • last_non_null_value/last_value support all data types.
  • listagg supports STRING data types.
  • bool_and/bool_or support BOOLEAN data type.

Note:

  • Pre-aggregate is only supported for table with primary key.
  • Pre-aggregate is not supported for streaming consuming.
  • Pre-aggregate currently only support INSERT changes.

Append-only Table #

Append-only tables are a performance feature that only accepts INSERT_ONLY data to append to the storage instead of updating or de-duplicating the existing data, and hence suitable for use cases that do not require updates (such as log data synchronization).

Create Append-only Table #

By specifying the core option 'write-mode' to 'append-only', users can create an append-only table as follows.

CREATE TABLE IF NOT EXISTS T1 (
    f0 INT,
    f1 DOUBLE,
    f2 STRING
) WITH (
    'write-mode' = 'append-only',
    'bucket' = '1' --specify the total number of buckets
)

Note:

  • By definition, users cannot define primary keys on an append-only table.
  • Append-only table is different from a change-log table which does not define primary keys. For the latter, updating or deleting the whole row is accepted, although no primary key is present.

Query Append-only Table #

Table Store supports reading append-only table with preserved sequential order within each bucket.

For example, with following write

INSERT INTO T1 VALUES
(1, 1.0, 'AAA'), (2, 2.0, 'BBB'), 
(3, 3.0, 'CCC'), (1, 1.0, 'AAA')

The query

SELECT * FROM T1

will return exactly the order of (1, 1.0, 'AAA'), (2, 2.0, 'BBB'), (3, 3.0, 'CCC'), (1, 1.0, 'AAA') because the total number of bucket is 1, and all records falls into the same bucket.

If we create another table with more than one bucket and specify the bucket key

CREATE TABLE IF NOT EXISTS T2 (
    f0 INT,
    f1 DOUBLE,
    f2 STRING
) WITH (
    'write-mode' = 'append-only',
    'bucket' = '2',
    'bucket-key' = 'f0'
)

The following write will write (1, 1.0, 'AAA'), (2, 2.0, 'BBB'), (1, 1.0, 'AAA') to bucket-0 and (3, 3.0, 'CCC') to bucket-1

INSERT INTO T2 VALUES
(1, 1.0, 'AAA'), (2, 2.0, 'BBB'), 
(3, 3.0, 'CCC'), (1, 1.0, 'AAA')

The query

SELECT * FROM T2

will return either (1, 1.0, 'AAA'), (2, 2.0, 'BBB'), (1, 1.0, 'AAA'), (3, 3.0, 'CCC') or (3, 3.0, 'CCC'), (1, 1.0, 'AAA'), (2, 2.0, 'BBB'), (1, 1.0, 'AAA').

Users can refer to Flink Data Types.

Note: MULTISET is not supported for all write-mode, and MAP is only supported as a non-primary key field in a primary-keyed table.