{1 2},{2 3},{3 4}

{1 2 3},{2 3 4}

{1 2 3 4}

(other possibilities will be excluded for lack of join clauses)

SELECT *

FROM tab1, tab2, tab3, tab4

{1 2 3},{1 3 4},{1 2 4}

{1 2 3 4}

In each of these examples, other join-order possibilities will be excluded

for lack of join clauses. (In reality, use of EquivalenceClasses would

allow us to deduce additional join clauses that allow more join orders,

but here we ignore that to preserve the simplicity of the examples.)

We consider left-handed plans (the outer rel of an upper join is a joinrel,

but the inner is always a single list item); right-handed plans (outer rel

is always a single item); and bushy plans (both inner and outer can be