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Oracle SQL WHERE clause Tips

Oracle Database Tips by Donald Burleson

Question:  I need to understand how to add an Oracle WHERE clause on my SQL.  Can you explain how the Oracle WHERE clause works?

Answer:  The Oracle WHERE clause is used to apply row filtering conditions to a SQL query, a way to limit the number of rows returned by the query.  re are a few important things to know

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There are some important caveats about the Oracle WHERE clause.

One of the most common Oracle SQL tuning problems are full-table scans that are imposed by invalidating an Oracle WHERE clause predicate with an Oracle built-in Function (BIF). This problem is especially prevalent when constraining SQL queries for date ranges because of the intrinsic encryption of the relational database DATE datatype.

Here are details on using function based indexes.

For example, these Oracle WHERE clause predicates might invoke an unexpected full-table scan:

WHERE trunc(ship_date) > trunc(sysdate-7);

WHERE to_char(ship_date,'YYYY-MM-DD') = '2004-01-04';

Oracle WHERE clause order can be important to performance! (only in 10g and previous releases)

Many people believe that the Oracle cost-based SQL optimizer does not consider the order that the Boolean predicates appear in the Oracle WHERE clause.

However, there is some evidence that this is not complete true, as evidenced by the ordered_predicates SQL hint.

The ordered_predicates hint (deprecated in 10g r2) is specified in the WHERE clause of a query and is used to specify the order in which Boolean predicates should be evaluated. In the absence of ordered_predicates (which is deprecated in Oracle 10g and beyond), Oracle uses the following steps to evaluate the order of SQL predicates:

  • Subqueries are evaluated before the outer Boolean conditions in the WHERE clause.
  • All Boolean conditions without built-in functions or subqueries are evaluated in reverse from the order they are found in the WHERE clause, with the last predicate being evaluated first.
  • Boolean predicates with built-in functions of each predicate are evaluated in increasing order of their estimated evaluation costs.

Why is searching for large-table full-table scans critical to SQL tuning?

If the optimizer gets confused or cannot find an appropriate index that matches the WHERE clause, the optimizer will read every row in the table. Hence large-table full-table scans often indicate a missing index or a sub-optimal choice of optimizer goal.

What is the relationship between indexes and SQL performance?

The sole purpose of indexes is to make SQL queries run faster. If the optimizer detects an indexes that matches part of the Oracle WHERE clause of the query, then the optimizer will use the index to avoid having to read every row in the table.

Oracle always interrogates the Oracle WHERE clause of the SQL statement to see if a matching index exists and then evaluates the cost to see of the index is the lowest-cost access method.

By using function-based indexes, the Oracle designer can create a matching index that exactly matches the predicates within the SQL where clause. This ensures that the query is retrieved with a minimal amount of disk I/O and the fastest possible speed. 

If you experiment with changing the order of predicates in the WHERE clause you will notice changes to the execution plan.

Also see the notes on the Oracle where clause:

Oracle where clause order can change performance

Oracle WHERE clause in export operations



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