Demand-Driven Query Execution

Volcano SQL Iterator Tree

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SELECT p.name, o.quantity FROM orders o

JOIN products p ON o.prod_id = p.id WHERE o.quantity > 5;

open() → initializes children

next() → parent demand

close() → releases state

yield count: 0

Phase 1 · Plan Tree

1. Closed iterator tree

Why This Matters

Component Inspector

Project

Root Operator · [name, quantity]

The root asks for one result at a time via next() and projects the final client columns without materializing intermediate tables.

Input Pull

Joined tuple from Hash Join

Output Yield

[name, quantity] to Client

Interactive Runtime Config