Relational Operations

1. Relational Operations • We will consider how to implement: • Selection ( ) Selects a subset of rows from relation. • Projection ( ) Deletes unwanted columns from relation. • Join ( ) Allows us to combine two relations. • Set-difference Tuples in reln. 1, but not in reln. 2. • UnionTuples in reln. 1 and in reln. 2. • Aggregation (SUM, MIN, etc.) and GROUP BY • Since each op returns a relation, ops can be composed! Later we discuss how to optimize expressions composed of several operators.

2. Schema for Examples Sailors (sid: integer, sname: string, rating: integer, age: real) Reserves (sid: integer, bid: integer, day: dates, rname: string) • Reserves: • Each tuple is 40 bytes long, 100 tuples per page, 1000 pages (I.e., 100,000 tuples, 4MB for the entire relation). • Sailors: • Each tuple is 50 bytes long, 80 tuples per page, 500 pages (I.e, 40,000 tuples, 2MB for the entire relation).

3. SELECT * FROM Reserves R WHERE R.rname < ‘C%’ Simple Selections • Of the form • With no index, unsorted: Must essentially scan the whole relation; cost is M (#pages in R). • With an index on selection attribute: Use index to find qualifying data entries, then retrieve corresponding data records. (Hash index useful only for equality selections.) • Result size estimation: (Size of R) * reduction factor. More on this later.

4. Using an Index for Selections • Cost depends on #qualifying tuples, and clustering. • Cost of finding qualifying data entries (typically small) plus cost of retrieving records. • In example, assuming uniform distribution of names, about 10% of tuples qualify (100 pages, 10000 tuples). With a clustered index, cost is little more than 100 I/Os; if unclustered, upto 10000 I/Os! • Important refinement for unclustered indexes: 1. Find sort the rid’s of the qualifying data entries. 2. Fetch rids in order. This ensures that each data page is looked at just once (though # of such pages likely to be higher than with clustering).

5. General Selection Conditions • (day<8/8/94 AND rname=‘Paul’) OR bid=5 OR sid=3 • Such selection conditions are first converted to conjunctive normal form (CNF):(day<8/8/94 OR bid=5 OR sid=3 ) AND (rname=‘Paul’ OR bid=5 OR sid=3) • We only discuss the case with no ORs (a conjunction of terms of the form attr op value). • An index matches (a conjunction of) terms that involve only attributes in a prefix of the search key. • Index on <a, b, c> matches a=5 AND b= 3, but notb=3.

6. Two Approaches to General Selections • First approach:Find the most selective access path, retrieve tuples using it, and apply any remaining terms that don’t match the index: • Most selective access path: An index or file scan that we estimate will require the fewest page I/Os. • Consider day<8/8/94 AND bid=5 AND sid=3: • A B+ tree index on day can be used; then, bid=5 and sid=3 must be checked for each retrieved tuple. • Similarly, a hash index on <bid, sid> could be used; day<8/8/94 must then be checked.

7. Intersection of Rids • Second approach • Get sets of rids of data records using each matching index. • Then intersect these sets of rids. • Retrieve the records and apply any remaining terms. • Consider day<8/8/94 AND bid=5 AND sid=3. • If we have a B+ tree index on day and an index on sid, we can retrieve rids of records satisfying day<8/8/94 using the first, rids of recs satisfying sid=3 using the second, intersect, retrieve records and check bid=5.

8. Implementing Projection SELECTDISTINCT R.sid, R.bid FROM Reserves R • Two parts: (1) remove unwanted attributes, (2) remove duplicates from the result. • Refinements to duplicate removal: • If an index on a relation contains all wanted attributes, then we can do an index-only scan. • If the index contains a subset of the wanted attributes, you can remove duplicates locally.

9. Equality Joins With One Join Column SELECT * FROM Reserves R1, Sailors S1 WHERE R1.sid=S1.sid • R S is a common operation. The cross product is too large. Hence, performing R S and then a selection is too inefficient. • Assume: M tuples in R, pR tuples per page, N tuples in S, pS tuples per page. • In our examples, R is Reserves and S is Sailors. • Cost metric: # of I/Os. We will ignore output costs.

10. Simple Nested Loops Join foreach tuple r in R do foreach tuple s in S do if ri == sj then add <r, s> to result • For each tuple in the outer relation R, we scan the entireinner relation S. • Cost: M + pR * M * N = 1000 + 100*1000*500 I/Os. • Page-oriented Nested Loops join: For each page of R, get each page of S, and write out matching pairs of tuples <r, s>, where r is in R-page and S is in S-page. • Cost: M + M*N = 1000 + 1000*500

11. Index Nested Loops Join foreach tuple r in R do foreach tuple s in S where ri == sj do add <r, s> to result • If there is an index on the join column of one relation (say S), can make it the inner. • Cost: M + ( (M*pR) * cost of finding matching S tuples) • For each R tuple, cost of probing S index is about 1.2 for hash index, 2-4 for B+ tree. Cost of then finding S tuples depends on clustering. • Clustered index: 1 I/O (typical), unclustered: upto 1 I/O per matching S tuple.

12. Examples of Index Nested Loops • Hash-index on sid of Sailors (as inner): • Scan Reserves: 1000 page I/Os, 100*1000 tuples. • For each Reserves tuple: 1.2 I/Os to get data entry in index, plus 1 I/O to get (the exactly one) matching Sailors tuple. Total: 220,000 I/Os. • Hash-index on sid of Reserves (as inner): • Scan Sailors: 500 page I/Os, 80*500 tuples. • For each Sailors tuple: 1.2 I/Os to find index page with data entries, plus cost of retrieving matching Reserves tuples. Assuming uniform distribution, 2.5 reservations per sailor (100,000 / 40,000). Cost of retrieving them is 1 or 2.5 I/Os depending on clustering.

13. . . . Block Nested Loops Join • Use one page as an input buffer for scanning the inner S, one page as the output buffer, and use all remaining pages to hold ``block’’ of outer R. • For each matching tuple r in R-block, s in S-page, add <r, s> to result. Then read next R-block, scan S, etc. Join Result R & S Hash table for block of R (k < B-1 pages) . . . . . . Output buffer Input buffer for S

14. Sort-Merge Join (R S) i=j • Sort R and S on the join column, then scan them to do a ``merge’’ on the join column. • Advance scan of R until current R-tuple >= current S tuple, then advance scan of S until current S-tuple >= current R tuple; do this until current R tuple = current S tuple. • At this point, all R tuples with same value and all S tuples with same value match; output <r, s> for all pairs of such tuples. • Then resume scanning R and S. • R is scanned once; each S group is scanned once per matching R tuple.

15. Example of Sort-Merge Join • Cost: M log M + N log N + (M+N) • The cost of scanning, M+N, could be M*N (unlikely!) • With 35, 100 or 300 buffer pages, both Reserves and Sailors can be sorted in 2 passes; total: 7500.

16. Original Relation Partitions OUTPUT 1 1 2 INPUT 2 hash function h . . . B-1 B-1 B main memory buffers Disk Disk Partitions of R & S Join Result Hash table for partition Ri (k < B-1 pages) hash fn h2 h2 Output buffer Input buffer for Si B main memory buffers Disk Disk Hash-Join • Partition both relations using hash fn h: R tuples in partition i will only match S tuples in partition i. • Read in a partition of R, hash it using h2 (<> h!). Scan matching partition of S, search for matches.

17. Cost of Hash-Join • In partitioning phase, read+write both relns; 2(M+N). In matching phase, read both relns; M+N I/Os. • In our running example, this is a total of 4500 I/Os. • Sort-Merge Join vs. Hash Join: • Given a minimum amount of memory both have a cost of 3(M+N) I/Os. Hash Join superior on this count if relation sizes differ greatly. Also, Hash Join shown to be highly parallelizable. • Sort-Merge less sensitive to data skew; result is sorted.