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The use_nl Hint
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The use_nl hint forces a nested loop join against the target tables. Unlike the other join hints that specify both tables, the use_nl hint only requires the name of the driving table (the first table in the from clause when using the CBO). The use_nl hint is seldom used in SQL tuning because both the CBO and the RBO tend to favor nested loop joins over hash or merge joins. However, the use_nl hint is sometimes useful for changing the driving table without changing the order of the tables in the from clause. This is because it is not always evident which table should be the driving table. Sometimes both tables have a comparable distribution of rows, and you can often improve the performance of a nested loop join by changing the driving table for the join. For more details on the use_nl hint, see Chapter 12.

The Hash Join

The hash join was first introduced in Oracle7.3 as an alternative to nested loop joins. The hash join technique improves the join speed of equi-joins by loading the driving table into RAM and using a hash technique to join into the second table.

A hash join is the recommended join method when the tables are different sizes and the smaller table is close to the available memory in hash_area_size. The basic premise of any hash join algorithm is to build an in-memory hash table from the smaller of the input row sources, the build input, and use the larger input to probe the hash table (Figure 16-7).

Figure 7: A hash join

Hash join algorithms work well for simple hash joins when the available hash area memory is large enough to hold the build input, but it is not necessary to fit entire probe input in hash_area_size memory. If the smaller driving table (the first table in the from clause) in a hash join does not fit into hash_area_size, Oracle will partition the hash probes and use temporary segments in the TEMP tablespace to manage the overflow.

TIP: When the smaller driving table is small enough to fit entirely into the RAM allocated by hash_area_size, then the hash join will generally perform faster than a nested loop join. Remember, you can adjust the hash_area_size for your query at the session level using the alter session command. Oracle recommends that the hash_area_size for the driving table be set at 1.6 times the sum of bytes for the driving table.

Internally, a hash join involves two phases, the partition phase and the join phase. To fully understand hash joins, let’s examine each of these phases.

The Partitioning Phase of a Hash Join

When the driving table cannot fit entirely into the RAM allocated by hash_area_size, a hash partition occurs. In a partition, contiguous pieces of the driving table are split into partitions on temporary segments in the TEMP tablespace (Figure 16-8).

Figure 8: Hash query partitioning

This partitioning effectively divides the hash query into many smaller inputs that can be independently processed. However, the problem of partitioning the inputs is not trivial. It is difficult to have a partitioning scheme that will split any data distribution into equal partitions without any skew. To minimize any skew in the partitioning, Oracle relies on column histograms and special bit-vector filtering techniques.

If, after partitioning, the smaller of the two inputs is larger than the size of the memory available to build the hash table, the hash table overflow is dealt with by performing a nested-loops hash join. The hash table is created  with the build input partition and then the  probe phase joins the tables. Then, the remainder of the build input is iteratively retrieved, and the hash table is built and joined with all the probe partitions until all of the build input is consumed.

The Join Phase of a Hash Join

Partition pairs of the build and probe inputs with the same key values are then joined (in what is called the join phase). This algorithm, known as the grace join, dramatically reduces the search space and key comparisons required for doing the join.

A limitation of the hash join algorithm is that it is based on the assumption that the distribution of join values in the tables is not skewed and that each partition receives approximately the same number of rows. This is generally not true and partition skew is a reality that needs to be dealt with.


This is an excerpt from "Oracle High-Performance SQL Tuning" by Donald K. Burleson, published by Oracle Press.

  
 

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