10.2.1.7 Nested-Loop Join Algorithms

MySQL executes joins between tables using a nested-loop algorithm or variations on it.

• Nested-Loop Join Algorithm

• Block Nested-Loop Join Algorithm

Nested-Loop Join Algorithm

A simple nested-loop join (NLJ) algorithm reads rows from the first table in a loop one at a time, passing each row to a nested loop that processes the next table in the join. This process is repeated as many times as there remain tables to be joined.

Assume that a join between three tables t1, t2, and t3 is to be executed using the following join types:

Table   Join Type
t1      range
t2      ref
t3      ALL

If a simple NLJ algorithm is used, the join is processed like this:

for each row in t1 matching range {
  for each row in t2 matching reference key {
    for each row in t3 {
      if row satisfies join conditions, send to client
    }
  }
}

Because the NLJ algorithm passes rows one at a time from outer loops to inner loops, it typically reads tables processed in the inner loops many times.

Block Nested-Loop Join Algorithm

A Block Nested-Loop (BNL) join algorithm uses buffering of rows read in outer loops to reduce the number of times that tables in inner loops must be read. For example, if 10 rows are read into a buffer and the buffer is passed to the next inner loop, each row read in the inner loop can be compared against all 10 rows in the buffer. This reduces by an order of magnitude the number of times the inner table must be read.

Prior to MySQL 8.0.18, this algorithm was applied for equi-joins when no indexes could be used; in MySQL 8.0.18 and later, the hash join optimization is employed in such cases. Starting with MySQL 8.0.20, the block nested loop is no longer used by MySQL, and a hash join is employed for in all cases where the block nested loop was used previously. See Section 10.2.1.4, “Hash Join Optimization”.

MySQL join buffering has these characteristics:

• Join buffering can be used when the join is of type ALL or index (in other words, when no possible keys can be used, and a full scan is done, of either the data or index rows, respectively), or range. Use of buffering is also applicable to outer joins, as described in Section 10.2.1.12, “Block Nested-Loop and Batched Key Access Joins”.

• A join buffer is never allocated for the first nonconstant table, even if it would be of type ALL or index.

• Only columns of interest to a join are stored in its join buffer, not whole rows.

• The join_buffer_size system variable determines the size of each join buffer used to process a query.

• One buffer is allocated for each join that can be buffered, so a given query might be processed using multiple join buffers.

• A join buffer is allocated prior to executing the join and freed after the query is done.

For the example join described previously for the NLJ algorithm (without buffering), the join is done as follows using join buffering:

for each row in t1 matching range {
  for each row in t2 matching reference key {
    store used columns from t1, t2 in join buffer
    if buffer is full {
      for each row in t3 {
        for each t1, t2 combination in join buffer {
          if row satisfies join conditions, send to client
        }
      }
      empty join buffer
    }
  }
}

if buffer is not empty {
  for each row in t3 {
    for each t1, t2 combination in join buffer {
      if row satisfies join conditions, send to client
    }
  }
}

If S is the size of each stored t1, t2 combination in the join buffer and C is the number of combinations in the buffer, the number of times table t3 is scanned is:

(S * C)/join_buffer_size + 1

The number of t3 scans decreases as the value of join_buffer_size increases, up to the point when join_buffer_size is large enough to hold all previous row combinations. At that point, no speed is gained by making it larger.