Define interleaved tables in Google Spanner?

An interleaved table is a child table whose rows are physically stored next to the parent row they belong to, based on a shared primary key prefix. You declare the relationship with INTERLEAVE IN PARENT, and the child's primary key must start with the parent's primary key columns.

CREATE TABLE Orders (
  CustomerId INT64 NOT NULL,
  OrderId INT64 NOT NULL,
  OrderDate DATE
) PRIMARY KEY (CustomerId, OrderId),
  INTERLEAVE IN PARENT Customers ON DELETE CASCADE;

Because parent and child rows sit in the same storage split, fetching a customer and their orders together avoids extra network hops, which is far cheaper than a traditional join across separately sharded tables. It also lets Spanner co-locate related data as it splits and moves ranges for load balancing.

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