1. Processing Models

A DBMS's processing model defines how the system executes a query plan.

<aside> 💡 Plan Processing Direction

Approach #1: Top-to-Bottom

• Start with the root and "pull" data up from its children.
• Tuples are always passed with function calls.

Approach #2: Bottom-to-Top

• Start with leaf nodes and push data to their parents.
• Allows for tighter control of caches/registers in pipelines. </aside>

1.1 Iterator Model

Also called Volcano or Pipeline Model.

Used in almost every DBMS. Allows for tuple pipelining.

Each query plan operator implements a Next() function.

• On each invocation, the operator returns either a single tuple or a null marker if there are no more tuples.
• The operator implements a loop that calls Next() on its children to retrieve their tuples and then process them.

<aside> 💡 Pipeline Breaker

Some operators must block until their children emit all their tuples.

→ Joins, Subqueries, Order By

</aside>

1.2 Materialization Model

Better for OLTP workloads because queries only access a small number of tuples at a time.

• Lower execution / coordination overhead.
• Fewer function calls.

Not good for OLAP queries with large intermediate results.

Each operator processes its input all at once and then emits its output all at once.