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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Nested loop join | 1/1 | https://en.wikipedia.org/wiki/Nested_loop_join | reference | science, encyclopedia | 2026-05-05T11:36:41.498435+00:00 | kb-cron |
A nested loop join is a naive algorithm that joins two relations by using two nested loops. Join operations are important for database management.
== Algorithm == Two relations
R
{\displaystyle R}
and
S
{\displaystyle S}
are joined as follows:
algorithm nested_loop_join is for each tuple r in R do for each tuple s in S do if r and s satisfy the join condition then yield tuple <r,s>
This algorithm will involve nr*bs+ br block transfers and nr+br seeks, where br and bs are number of blocks in relations R and S respectively, and nr is the number of tuples in relation R. The algorithm runs in
O
(
|
R
|
|
S
|
)
{\displaystyle O(|R||S|)}
I/Os, where
|
R
|
{\displaystyle |R|}
and
|
S
|
{\displaystyle |S|}
is the number of tuples contained in
R
{\displaystyle R}
and
S
{\displaystyle S}
respectively and can easily be generalized to join any number of relations ... The block nested loop join algorithm is a generalization of the simple nested loops algorithm that takes advantage of additional memory to reduce the number of times that the
S
{\displaystyle S}
relation is scanned. It loads large chunks of relation R into main memory. For each chunk, it scans S and evaluates the join condition on all tuple pairs, currently in memory. This reduces the number of times S is scanned to once per chunk.
== Index join variation == If the inner relation has an index on the attributes used in the join, then the naive nest loop join can be replaced with an index join.
algorithm index_join is for each tuple r in R do for each tuple s in S in the index lookup do yield tuple <r,s>
The time complexity for this variation improves from
O
(
|
R
|
|
S
|
)
to
O
(
|
R
|
log
|
S
|
)
{\displaystyle O(|R||S|){\text{ to }}O(|R|\log |S|)}
== See also == Hash join Sort-merge join
== References ==