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HASH JOIN RIGHT ANTI

Short Description

Detailed Description​

A HASH JOIN RIGHT ANTI in an Oracle execution plan is a specific type of hash join operation.

Here’s what it means:

  1. HASH JOIN:
    1. A join method where Oracle uses a hash table to efficiently match rows between two datasets.
    2. Typically used for joining large tables or when indexes are not available or optimal.
  2. RIGHT:
    1. Refers to the order of the join.
    2. A RIGHT join means that the second (right-hand) table in the execution plan is treated as the outer table during the join operation.
  3. ANTI:
    1. Indicates an anti-join.
    2. An anti-join finds rows in one dataset that do not have matching rows in the other dataset.
    3. Often used to implement queries with conditions like NOT IN or NOT EXISTS.

How It Works:

  1. Oracle builds a hash table for the smaller dataset (in this case, the right table).
  2. Oracle scans the larger dataset (the left table) and checks each row against the hash table.
  3. Rows from the left table that do not match any rows in the hash table are returned.

Why It’s Used:

  • Efficiency: Hash joins are generally efficient for large datasets, especially when indexes are absent.
  • Anti-join optimisation: Helps to optimise NOT IN or NOT EXISTS queries by avoiding row-by-row comparison.

If performance is poor, consider:

  • Ensuring indexes on the relevant columns.
  • Using query hints (e.g., USE_NL for nested loops if data distribution supports it).
  • Checking table statistics and refreshing them if outdated.The choice of HASH JOIN RIGHT ANTI indicates Oracle is optimising the query based on the data distribution and join condition.

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