Model

Join size estimate

Exact when every key value finds matches and rows spread evenly over the distinct values; the assumptions fail on skewed and correlated keys.

∣ A ⋈ B ∣ \approx \frac{∣ A ∣ \times ∣ B ∣}{max ( V ( A , a ) , V ( B , b ) )}

$V (A, a)$: Distinct values; the number of distinct values of column a in table A

Worked examples, recomputed

Every number the mechanisms quote for this relationship is recomputed from it, so none of them disagrees with it.

Each mechanism states it in its own names. The general symbol each name stands for is shown with it.

Query Planning & Optimization
How does a system choose a plan for a question when the facts that decide the best plan are unknown until the plan has run?
$∣ A ⋈ B ∣ = ∣ A ∣ \times ∣ B ∣/ max (V (A, a), V (B, b))$
Read it in place →