Stage 4: Aggregations (COUNT, SUM, AVG, MIN, MAX)

Concept

Implement aggregate functions that compute summary values over sets of rows, producing a single summary row for the entire table.

What It Teaches

• Aggregate PlanNode: A node that consumes all rows from its child, computes aggregate values, and emits a single summary row.
• Accumulators: Each aggregate function (COUNT, SUM, AVG, MIN, MAX) maintains an accumulator that is updated for each input row.

Learning Objectives

1. Implement COUNT(*), COUNT(column), SUM(column), AVG(column), MIN(column), MAX(column).
2. Implement an AggregateNode PlanNode that consumes all child rows and produces summary output.
3. Handle NULL values correctly in aggregates (COUNT skips NULLs, SUM treats NULL as 0).

New SQL Syntax

SELECT COUNT(*) FROM users;
SELECT AVG(id) FROM users;
SELECT MIN(id), MAX(id) FROM users;

Explain Output

[PLAN] SELECT COUNT(*) FROM users
[PLAN]   └── Aggregate (aggs=[COUNT(*)])
[PLAN]       └── SeqScan (table=users)