When Would A Cost-based Optimizer Not Make A Right Decision And Would Then Need To Be Overridden?

What is the difference between a rule-based optimizer and a cost-based optimizer?

The rule-based optimizer (RBO) is the original optimizer to the Oracle database. The cost-based optimizer (CBO) originated back in Oracle7 but became more popular as databases increased in size. RBO follows a set of rules mostly based on indexes and types of indexes.

What type s of queries benefit the most from cost-based query optimization?

Cost-based optimization is always used with parallel query and with partitioned tables.

What is cost-based optimizer in Oracle?

The Oracle cost-based optimizer (CBO) displays the cost number for a query, or an estimate based on statistics and calculations. The cost number is the estimated number of physical I/O operations Oracle thinks it will have to find the requested data, based solely on statistics.

What are the approaches used by optimizer during execution plan?

The Optimizer uses costing methods, cost-based optimizer (CBO), or internal rules, rule-based optimizer (RBO), to determine the most efficient way of producing the result of the query. The Row Source Generator receives the optimal plan from the optimizer and outputs the execution plan for the SQL statement.

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What is a rule-based optimizer?

These are the optimization strategies used by Database engines for executing a query or a stored procedure. They come into picture after a query or Stored Procedure is compiled and is just about to execute (most databases also cache these generated execution plans).

What are optimizer hints give an example of their use?

Hints provide a mechanism to instruct the optimizer to choose a certain query execution plan based on the specific criteria. For example, you might know that a certain index is more selective for certain queries. Based on this information, you might be able to choose a more efficient execution plan than the optimizer.

What is query optimization with example?

Query optimization is the overall process of choosing the most efficient means of executing a SQL statement. SQL is a nonprocedural language, so the optimizer is free to merge, reorganize, and process in any order. The database optimizes each SQL statement based on statistics collected about the accessed data.

What is cost based query optimization?

Cost-based query optimization compares different strategies based on relative costs (amount of time that the query needs to run) and selects and executes one that minimizes the cost. The cost of a strategy is just an estimate based on how many estimated CPU and I/O resources that the query will use.

What are the different ways to optimize a SQL query?

Supercharge Your SQL Queries for Production Databases

  • Define business requirements first.
  • SELECT fields instead of using SELECT *
  • Avoid SELECT DISTINCT.
  • Create joins with INNER JOIN (not WHERE)
  • Use WHERE instead of HAVING to define filters.
  • Use wildcards at the end of a phrase only.
  • Use LIMIT to sample query results.
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What is cost of query in Oracle?

Cost is the estimated amount of work the plan will do. A higher cardinality => you’re going to fetch more rows => you’re going to do more work => the query will take longer. Thus the cost is (usually) higher. All other things being equal, a query with a higher cost will use more resources and thus take longer to run.

How explain plan works in Oracle?

To explain a SQL statement, use the EXPLAIN PLAN FOR clause immediately before the statement. For example: EXPLAIN PLAN FOR SELECT last_name FROM employees; This explains the plan into the PLAN_TABLE table.

Why do we use triggers in Oracle?

A trigger is a named PL/SQL block stored in the Oracle Database and executed automatically when a triggering event takes place. These triggers are often used for auditing purposes to record changes of the schema. A system event such as startup or shutdown of the Oracle Database. A user event such as login or logout.

Which operation are allowed in a join view?

Set operations, such as UNION, UNION ALL, INTERSECT, MINUS. Aggregate functions, such as AVG, COUNT, MAX, MIN, SUM, and so forth.

How do you optimize a join in Oracle?

Nested Loops Join

  1. The optimizer chooses one of the tables as the outer table, or the driving table.
  2. For each row in the outer table, Oracle finds all rows in the inner table that satisfy the join condition.
  3. Oracle combines the data in each pair of rows that satisfy the join condition and returns the resulting rows.
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What is reduce cost in Explain Plan in Oracle?

The cost of the plan is reduced by rewriting the SQL statement to use an EXISTS. This plan is more effective, because two indexes are used to satisfy the predicates in the parent query, thus returning only a few employee_ids. The employee_ids are then used to access the orders table through an index.

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