Query Execution

1. Query Execution One-pass algorithm for database operations Chetan Sharma 008565661

2. Overview • One-Pass Algorithm • One-Pass Algorithm Methods: • Tuple-at-a-time, unary operations. • Full-relation, unary operations. • Full-relation, binary operations.

3. we can divide algorithms for operators into three “degrees” of difficulty and cost: 1) Require at least one of the arguments to fit in main memory.-one pass 2) Some methods work for data that is too large to fit in available main memory but not for the largest imaginable data sets.-two pass 3) Some methods work without a limit on the size of the data.-multipass:recursivegeneralizations of the two-pass algorithms.

4. One-Pass Algorithm Reading the data only once from disk. Usually, they require at least one of the arguments to fit in main memory

5. Tuple-at-a-Time These operations do not require an entire relation, or even a large part of it, in memory at once. Thus, we can read a block at a time, use one main memorybuffer, and produce our output. Ex- selection and projection

6. Tuple-at-a-Time A selection or projection being performed on a relation R

7. Full-relation, unary operations Now, let us consider the unary operations that apply to relations as a whole , rather than to one tuple at a time: a)Duplicate elimination. b)Grouping .

8. a) Duplicate elimination

9. b) Grouping MIN (a),MAX (a) aggregate, record the minimum or maximum value, respectively, of attribute a seen for any tuple in the group so far. COUNT aggregation, add one for each tuple of the group that is seen. SUM (a), add the value of attribute a to the accumulated sum for its group. AVG (a) is the hard case. We must maintain two accumulations: the count of the number of tuples in the group and the sum of the a-values of these tuples.

10. b) Grouping When all tuples of R have been read into the input buffer and contributed to the aggregation(s) for their group, we can produce the output by writing the tuple for each group. Note-: that until the last tuple is seen, we cannot begin to create output for aoperation. Thus, this algorithm does not fit the iterator framework very well; The entire grouping has to be done by the Open method before the first tuple can be retrieved

11. One-Pass Algorithms for Binary Operations All other operations are in this class: set and bag versions of union, intersection, difference, joins, and products. binary operations require reading the smaller of the operands R and S into main memory and building a suitable data structure so tuples can be both inserted quickly and found quickly. to be performed in one pass is: min(B(R),B(S)) <= M

12. Some examples In each case, we assume R is the larger of the relations, and we house S in main memory. • Set Union: -We read S into M - 1 buffers of main memory and build a search structure where the search key is the entire tuple. -All these tuples are also copied to the output. -Read each block of R into the Mth buffer, one at a time. -For each tuple t of R, see if t is in S, and if not, we copy t to the output. If t is also in S, we skip t. • Set Intersection : -Read S into M - 1 buffers and build a search structure with full tuples as the search key. -Read each block of R, and for each tuple t of R, see if t is also in S. If so, copy t to the output, and if not, ignore t.

13. examples continued.. • Product • Read S into M — 1 buffers of main memory; no special data structure is needed. Then read each block of R, and for each tuple t of R concatenate t with each tuple of S in main memory. Output each concatenated tuple as it is formed.

14. Summery • One-Pass Algorithm • One-Pass Algorithm Methods: • Tuple-at-a-time, unary operations. • Full-relation, unary operations. • Full-relation, binary operations.

15. Questions &