1 / 11

QUERY EXECUTION

QUERY EXECUTION. 15.3 Nested-Loop Joins. By: Saloni Tamotia (215). Introduction to Nested-Loop Joins. Used for relations of any side. Not necessary that relation fits in main memory Uses “ One-and-a-half” pass method in which for each variation: One argument read just once.

gibson
Download Presentation

QUERY EXECUTION

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. QUERY EXECUTION 15.3 Nested-Loop Joins By: SaloniTamotia (215)

  2. Introduction to Nested-Loop Joins • Used for relations of any side. • Not necessary that relation fits in main memory • Uses “One-and-a-half” pass method in which for each variation: • One argument read just once. • Other argument read repeatedly. • Two kinds: • Tuple-Based Nested Loop Join • Block-Based Nested Loop Join

  3. ADVANTAGES OF NESTED-LOOP JOIN • Fits in the iterator framework. • Allows us to avoid storing intermediate relation on disk.

  4. Tuple-Based Nested-Loop Join • Simplest variation of the nested-loop join • Loop ranges over individual tuples

  5. Tuple-Based Nested-Loop Join • Algorithm to compute the Join R(X,Y) | | S(Y,Z) FOR each tuple s in S DO FOR each tuple r in R DO IF r and s join to make tuple t THEN output t • R and S are two Relations with r and s as tuples. • carelessness in buffering of blocks causes the use of T(R)T(S) disk I/O’s

  6. IMPROVEMENT & MODIFICATION To decrease the cost • Method 1: Use algorithm for Index-Based joins • We find tuple of R that matches given tuple of S • We need not to read entire relation R • Method 2: Use algorithm for Block-Based joins • Tuples of R & S are divided into blocks • Uses enough memory to store blocks in order to reduce the number of disk I/O’s.

  7. Block-Based Nested-Loop Join Algorithm • Access to arguments is organized by block. • While reading tuples of inner relation we use less number of I/O’s disk. • Using enough space in main memory to store tuples of relation of the outer loop. • Allows to join each tuple of the inner relation with as many tuples as possible.

  8. Block-Based Nested-Loop Join Algorithm ALGORITHM: FOR each chunk of M-1 blocks of SDO FOR each block b of RDO FOR each tuplet of bDO find the tuples of S in memory that join with t output the join of t with each of these tuples

  9. Block-Based Nested-Loop Join Algorithm • Assumptions: • B(S) ≤ B(R) • B(S) > M This means that the neither relation fits in the entire main memory.

  10. Analysis of Nested-Loop Join • Number of disk I/O’s: [B(S)/(M-1)]*(M-1 +B(R)) or B(S) + [B(S)B(R)/(M-1)] or approximately B(S)*B(R)/M

  11. QUESTIONS

More Related