INTERLEAVE IN PARENT

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Interleaving tables improves query performance by optimizing the key-value structure of closely related tables, attempting to keep data on the same key-value range if it's likely to be read and written together.

How Interleaved Tables Work

When tables are interleaved, data written to one table (known as the child) is inserted directly into another (known as the parent) in the key-value store. This is accomplished by matching the child table's Primary Key to the parent's.

Interleave Prefix

For interleaved tables to have Primary Keys that can be matched, the child table must use the parent table's entire Primary Key as a prefix of its own Primary Key––these matching columns are referred to as the interleave prefix. It's easiest to think of these columns as representing the same data, which is usually implemented with Foreign Keys.

For example, if you want to interleave orders into customers and the Primary Key of customers is id, you need to create a column representing customers.id as the first column in the Primary Key of orders—e.g., with a column called customer. So the data representing customers.id is the interleave prefix, which exists in the orders table as the customer column.

Key-Value Structure

When you write data into the child table, it is inserted into the key-value store immediately after the parent table's key matching the interleave prefix.

For example, if you interleave orders into customers, the orders data is written directly within the customers table in the key-value store. The following is a crude, illustrative example of what the keys would look like in this structure:

/customers/1
/customers/1/orders/1000
/customers/1/orders/1002
/customers/2
/customers/2/orders/1001
/customers/2/orders/1003
...
/customers/n/
/customers/n/orders/<order belonging to n>

By writing data in this way, related data is more likely to remain on the same key-value range, which can make it much faster to read from and write to. Using the above example, all of customer 1's data is going to be written to the same range, including its representation in both the customers and orders tables.

When to Interleave Tables

You're most likely to benefit from interleaved tables when:

• Your tables form a hierarchy
• Queries maximize the benefits of interleaving
• Queries do not suffer too greatly from interleaving's tradeoffs

Interleaved Hierarchy

Interleaved tables typically work best when the tables form a hierarchy. For example, you could interleave the table orders (as the child) into the table customers (as the parent, which represents the people who placed the orders). You can extend this example by also interleaving the tables invoices (as a child) and packages (as a child) into orders (as the parent).

The entire set of these relationships is referred to as the interleaved hierarchy, which contains all of the tables related through interleave prefixes.

Benefits

In general, reads, writes, and joins of values related through the interleave prefix are much faster. However, you can also improve performance with any of the following:

• Filtering more columns in the interleave prefix (from left to right).
  
  For example, if the interleave prefix of packages is (customer, order), filtering on customer would be fast, but filtering on customer and order would be faster.
• Using only tables in the interleaved hierarchy.

Tradeoffs

• In general, reads and deletes over ranges of table values (e.g., WHERE column > value) in interleaved tables are slower.
  
  However, an exception to this is performing operations on ranges of table values in the greatest descendant in the interleaved hierarchy that filters on all columns of the interleave prefix with constant values.
  
  For example, if the interleave prefix of packages is (customer, order), filtering on the entire interleave prefix with constant values while calculating a range of table values on another column, like WHERE customer = 1 AND order = 1001 AND delivery_date > DATE '2016-01-25', would still be fast.
• If the amount of interleaved data stored for any Primary Key value of the root table is larger than a key-value range's maximum size (64MB by default), the interleaved optimizations will be diminished.
  
  For example, if one customer has 200MB of order data, their data is likely to be spread across multiple key-value ranges and CockroachDB will not be able to access it as quickly, despite it being interleaved.

Syntax

Parameters

Requirements

• You can only interleave tables when creating the child table.
• Each child table's Primary Key must contain its parent table's Primary Key as a prefix (known as the interleave prefix).
  
  For example, if the parent table's primary key is (a INT, b STRING), the child table's primary key could be (a INT, b STRING, c DECIMAL).
  Note:

  This requirement is enforced only by ensuring that the columns use the same data types. However, we recommend ensuring the columns refer to the same values by using the Foreign Key constraint.
• Interleaved tables cannot be the child of more than 1 parent table. However, each parent table can have many children tables. Children tables can also be parents of interleaved tables.

Recommendations

• Use interleaved tables when your schema forms a hierarchy, and the Primary Key of the root table (for example, a "user ID" or "account ID") is a parameter to most of your queries.
• To enforce the relationship between the parent and children table's Primary Keys, use Foreign Key constraints on the child table.
• In cases where you're uncertain if interleaving tables will improve your queries' performance, test how tables perform under load when they're interleaved and when they aren't.

Examples

Interleaving Tables

This example creates an interleaved hierarchy between customers, orders, and packages, as well as the appropriate Foreign Key constraints. You can see that each child table uses its parent table's Primary Key as a prefix of its own Primary Key (the interleave prefix).

> CREATE TABLE customers (
    id INT PRIMARY KEY,
    name STRING(50)
  );

> CREATE TABLE orders (
    customer INT,
    id INT,
    total DECIMAL(20, 5),
    PRIMARY KEY (customer, id),
    CONSTRAINT fk_customer FOREIGN KEY (customer) REFERENCES customers
    ) INTERLEAVE IN PARENT customers (customer)
  ;

> CREATE TABLE packages (
    customer INT,
    "order" INT,
    id INT,
    address STRING(50),
    delivered BOOL,
    delivery_date DATE,
    PRIMARY KEY (customer, "order", id),
    CONSTRAINT fk_order FOREIGN KEY (customer, "order") REFERENCES orders
    ) INTERLEAVE IN PARENT orders (customer, "order")
  ;

Key-Value Storage Example

It can be easier to understand what interleaving tables does by seeing what it looks like in the key-value store. For example, using the above example of interleaving orders in customers, we could insert the following values:

> INSERT INTO customers
  (id, name) VALUES
  (1, 'Ha-Yun'),
  (2, 'Emanuela');

> INSERT INTO orders
  (customer, id, total) VALUES
  (1, 1000, 100.00),
  (2, 1001, 90.00),
  (1, 1002, 80.00),
  (2, 1003, 70.00);

Using an illustrative format of the key-value store (keys are on the left; values are represented by -> value), the data would be written like this:

/customers/<customers.id = 1> -> 'Ha-Yun'
/customers/<orders.customer = 1>/orders/<orders.id = 1000> -> 100.00
/customers/<orders.customer = 1>/orders/<orders.id = 1002> -> 80.00
/customers/<customers.id = 2> -> 'Emanuela'
/customers/<orders.customer = 2>/orders/<orders.id = 1001> -> 90.00
/customers/<orders.customer = 2>/orders/<orders.id = 1003> -> 70.00

You'll notice that customers.id and orders.customer are written into the same position in the key-value store. This is how CockroachDB relates the two table's data for the interleaved structure. By storing data this way, accessing any of the orders data alongside the customers is much faster.

To better understand how CockroachDB writes key-value data, see our blog post Mapping Table Data to Key-Value Storage.

See Also

• CREATE TABLE
• Foreign Keys
• Column Families

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