The LATERAL keyword in SQL standard can precede a sub-SELECT FROM item. This allows the sub-SELECT to refer to columns of FROM items that appear before it in the FROM list. (Without LATERAL, each sub-SELECT is evaluated independently and so cannot cross-reference any other FROM item.)
Calcite supports the LATERAL syntax. In Drill, we should add support for it in the planning and execution phase.
The main motivation of supporting it is it makes it more expressive and performant to handling complex types such as arrays and maps. For instance, suppose you have a customer table which contains 1 row per customer containing customer-id, name and an array of Orders corresponding to each customer. Suppose you want to find out for each customer what is the average order amount. This could be expressed as follows using SQL standard LATERAL and UNNEST syntax:
The subquery may contain other operations such as filtering etc which operate on the output of the un-nested c.orders array. The UNNEST operation is supported in Drill today using FLATTEN operator. More details of the use cases for LATERAL is available from existing product documentations .. e.g see .