Nested Loop Join Algorithm Vlad Mihalcea This document discusses improvements to nested loop joins (nljs) in oracle 11g. a new technique called table batching is introduced that creates two nljs one for the index and another for the table. this performs similarly to the prefetching technique in 9i. For each tuple in the outer relation r, we scan the entire inner 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
Nested Loop Join Technique General rules for nested loop join use block nested loop join if no table fits into memory smaller table on the left in each iteration read as many blocks in the left (outer) table as possible. Improvements to nested loop and block nested loop algorithms: in block nested loop, use m — 2 disk pages as blocking unit for outer relations, where m = memory size in pages; use remaining two pages to buffer inner relation and output. 2 pages page based nested loop join is an optimization of tuple based nested loop join because it scans over s once for every page in r, while tuple based join scans s once for every tuple in r. as a result, it prevents s from being scanned as many times, thus decreasing the number of disk reads. We can then use the external merge sort algorithm to join the sorted tables. step through the two sorted tables with cursors and emit matching tuples. may need to backtrack depending on the join type.
Nested Loop Join Technique 2 pages page based nested loop join is an optimization of tuple based nested loop join because it scans over s once for every page in r, while tuple based join scans s once for every tuple in r. as a result, it prevents s from being scanned as many times, thus decreasing the number of disk reads. We can then use the external merge sort algorithm to join the sorted tables. step through the two sorted tables with cursors and emit matching tuples. may need to backtrack depending on the join type. V alduriez and gardarin [v g84 ] compared parallel join and semijoin algorithms based on hashing, sort merge, and nested lo ops, but did not consider nested lo ops with index. The simplest join algorithm is a nested loop that joins the two relations together a tuple at a time. the outer loop iterates over each tuple in one relation r, and the inner loop iterates over each tuple in the second relation s. 2 nested loop join at a high level, this type of join algorithm is comprised of two nested for loops that iterate over the tuples in both tables and compares each of them pairwise. if the tuples match the join predicate, then output them. 1. nested loop join: (nested loop join slide) for each r in r do for each s in s do if r.c = s.c then output r,s pair q: if r has 100,000 tuples, how many times the entire s table is scanned? the simplest algorithm. it works, but may not be e cient. 2. index join: (index join slide).
Nested Loop Join Technique Part2 Pdf V alduriez and gardarin [v g84 ] compared parallel join and semijoin algorithms based on hashing, sort merge, and nested lo ops, but did not consider nested lo ops with index. The simplest join algorithm is a nested loop that joins the two relations together a tuple at a time. the outer loop iterates over each tuple in one relation r, and the inner loop iterates over each tuple in the second relation s. 2 nested loop join at a high level, this type of join algorithm is comprised of two nested for loops that iterate over the tuples in both tables and compares each of them pairwise. if the tuples match the join predicate, then output them. 1. nested loop join: (nested loop join slide) for each r in r do for each s in s do if r.c = s.c then output r,s pair q: if r has 100,000 tuples, how many times the entire s table is scanned? the simplest algorithm. it works, but may not be e cient. 2. index join: (index join slide).
Nested Loop Join Technique Part2 Pdf 2 nested loop join at a high level, this type of join algorithm is comprised of two nested for loops that iterate over the tuples in both tables and compares each of them pairwise. if the tuples match the join predicate, then output them. 1. nested loop join: (nested loop join slide) for each r in r do for each s in s do if r.c = s.c then output r,s pair q: if r has 100,000 tuples, how many times the entire s table is scanned? the simplest algorithm. it works, but may not be e cient. 2. index join: (index join slide).
Nested Loop Join Technique Part2 Pdf
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