ONE PASS ALGORITHM PRESENTED BY: PRADHYUMAN RAOL ID : 114 Instructor: Dr T.Y. LIN

Agenda  One Pass algorithm  Tuple-at-a-Time  Unary Operators  Binary Operations

One-Pass Algorithms  One Pass Algorithm:  Some methods involve reading the data only once from disk.  They work only when at least one of the arguments of the operation fits in main memory.

Tuple-at-a-Time  We read the blocks of R one at a time into an input buffer, perform the operation on the tuple, and more the selected tuples or the projected tuples to the output buffer.  Examples: Selection & Projection

Tuple at a time Diagram Input bufferOutput buffer Unary operator R

Unary Operators  The unary operations that apply to relations as a whole, rather than to one tuple at a time.  Duplicate Elimination  (R) :Check whether that tuple is already there or not. M= Main memory B(  (R))= Size of Relation R Assumption: B(  (R)) <= M

Unary Operators  Grouping : A grouping operation gives us zero or more grouping attributes and presumably one or more accumulated value or values for each aggregation.  Min or Max  Count – For any count aggregation, add one for each tuple of the group that is seen.  Sum- add the value of attribute a to the accumulated sum for its group provided, a is not NULL.  Average

Binary Operations  Set Union- We read S into M-1 buffers of main memory and build a search structure whose search key is the entire tuple.  Set Intersection- Read S into M-1 buffers and build a search structure with full tuples as the search key.

Continued…  Set Difference  Bag Intersection  Bag Difference  Product  Natural Join- To compute the Natural Join we have to do the following: 1.Read all the tuples of S form them into a main memory search structures with the attributes of Y as the search key. Use M-1 block of memory for this purpose.

Continued…  2. Read each block of R into the one remaining main memory buffer. For each tuple t of R, find the tuples of S that agree with t on all attributes of Y, using the search Structure. For each matching tuple S, form a tuple by joining it with t, and move the resulting tuple to the output.

Thank You