How to rewrite Oracle OR

Oracle OR rewriting: Optimization from simple statements to complex queries

In Oracle database, the OR operator is a commonly used logical operator, used to connect two or more conditions, as long as one of them When one condition is true, the entire set of conditions is true. However, in complex queries, using the OR operator may cause query performance to decrease, so OR rewriting is particularly important. This article will introduce how to rewrite Oracle OR from the perspective of simple statements to complex queries to optimize query performance.

1. Application of OR rewriting in simple statements

In simple statements, we can optimize query performance by rewriting OR into UNION ALL. For example, the original query statement is:

SELECT * FROM t WHERE col1 = 'A' OR col2 = 'B';

is rewritten through OR as:

SELECT * FROM t WHERE col1 = 'A'
UNION ALL
SELECT * FROM t WHERE col2 = 'B';

In this way, the query will first execute two subqueries separately, and then merge the results, thereby reducing the OR operation The use of symbols improves query performance.

2. Application of OR rewriting in complex queries

In complex queries, the use of the OR operator may cause problems such as full table scan or index failure, resulting in Query performance degrades. Therefore, different OR rewriting methods need to be used to optimize query performance, as follows:

(1) Replace the OR operator by subquery

For example, the original query statement is:

SELECT * FROM t WHERE col1 = 'A' OR col2 = 'B' OR col3 = 'C';

Replacing the OR operator through subqueries is:

SELECT * FROM t WHERE col1 = 'A'
UNION
SELECT * FROM t WHERE col2 = 'B'
UNION
SELECT * FROM t WHERE col3 = 'C';

In this way, the query will first execute three subqueries separately, and then merge the results, avoiding the use of the OR operator and improving query performance. .

(2) Replace the OR operator through conditional reorganization

For example, the original query statement is:

SELECT * FROM t WHERE col1 = 'A' OR col2 = 'B' OR col3 = 'C' OR col4 = 'D';

Replace the OR operator through conditional reorganization:

SELECT * FROM t WHERE (col1 = 'A' OR col2 = 'B') AND (col3 = 'C' OR col4 = 'D');

In this way, the query will first reorganize the conditions into two sub-condition groups, and then execute the query, avoiding the use of the OR operator and improving query performance.

(3) Use the EXISTS operator to replace the OR operator

For example, the original query statement is:

SELECT * FROM t1 WHERE col1 = 'A' OR col2 = 'B' OR col3 = 'C';

Use the EXISTS operator to replace the OR operator:

SELECT * FROM t1
WHERE EXISTS (SELECT 1 FROM t2 WHERE t1.col1 = 'A' AND t2.col2 = t1.col2 AND t2.col3 = 'C')
   OR EXISTS (SELECT 1 FROM t2 WHERE t1.col2 = 'B' AND t2.col1 = t1.col1 AND t2.col3 = 'C');

In this way, the query will decompose the original query into two subqueries, and use the EXISTS operator to nest the subqueries, avoiding the use of the OR operator and improving query performance.

In short, OR rewriting can effectively optimize Oracle query performance, especially in complex query scenarios. The selection of various rewriting methods needs to be measured and chosen based on the actual situation to achieve optimal query performance.

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