1. Performance Join Operator Select * from R, S where R.a = S.a;
perform a cross product, followed by select and project
NO -- the result can be large.
What if three (or more) relations?
JOIN is a binary operator
Techniques:
J1. Nested loops (nested blocks) join
J2. Index nested loops join
J3. Sort-Merge join
J4. Partition-Hash join
Performance
11/14/18.1

2. J2. Index Nested Loops Join
If there is an index, take advantage of it. Instead of two table scans, use one table scan and one index scan
Does it matter if the index is the inner or outer loop?
foreach tuple r in R
foreach tuple s in S where ri = si
add (r,s) to result
Join selection factor: the fraction of records in one table that will be joined with records in the other table.
11/14/18.2

3. Index Nested Loops Join
foreach tuple r in R
foreach tuple s in S where ri = si
add (r,s) to result
S has an index with depth xS = 2
rR = 6000, bR = 2000, rS = 50, bS = 10
-- nested loop: index
bR + (rR * (xS + 1)) = (6000*(2+1)) = 20,000
-- outer loop: index
bS*(xS+1) * bR = 10 * (2+1) * 2000 = 60,000
11/14/18.3

4. Index Index Nested Loops Join
foreach tuple r in R
foreach tuple s in S where ri = si
add (r,s) to result
If R and S each have an index:
S index depth xS = 2 and R index depth xR = 4
rR = 6000, bR = 2000, rS = 50, bS = 10
-- R outer loop
bR + (rR * (xS + 1)) = (6000*(2+1)) = 20,000
-- S outer loop
bS + (rS * (xR + 1)) = 10 + (50*(4+1)) = 260
11/14/18.4