Oracle SQL Plus The Definitive Guide- P34 pot

Suppose that you frequently execute the following query, which searches for an employee by name and displays that employee's current billing rate: SELECT employee_name, employee_billing_

First, though, you have to be sure the default execution path is a bad one This isn't as easy as you may think.

Knowing Good Results from Bad

Knowing a good execution plan from a bad one requires some degree of experience and judgment It also helps to

understand your data In many cases, it may not be enough to look at the plan You may have to do some benchmarking

as well

Consider the issue of doing a full table scanreading all the rows in the tableto find rows for a query On the surface, reading the entire table to find the desired rows seems like a very inefficient approach Many people avoid it out of hand, thinking that an indexed retrieval is always better If you have a reasonably large table and are searching for only one or two rows, then a full table scan is not an efficient approach However, if you are retrieving or summarizing a large percentage of the rows in the table, then a full table scan will likely outperform an indexed retrieval The problem

is that somewhere in between these two extremes lies a large grey area That's where you have to do some

benchmarking and use some judgment based on your expectations of what the query will be asked to do when it is in production

Here are some questions to ask yourself as you look at an execution plan:

Is a table scan being used to retrieve only a small percentage of rows from a large table? If so, you may want to create

an index

Is an index scan being used when you are retrieving, or summarizing, a large percentage of a table's rows? If so, you may be better of forcing a full table scan

Is Oracle using the most selective index? An index on a YES/NO field would typically be much less selective than an index on last name, for example

Is Oracle joining the largest table last? It's generally better to eliminate as many rows as possible prior to any joins

You should be able to make changes based on the above assumptions and be reasonably certain you will gain a

performance improvement It's always best, however, to test and validate that an improvement has occurred

Creating Indexes

Creating indexes is an easy way to affect a query If, for example, you have a large employee table, much larger than the one used in this book, keyed on employeeid, and your query is searching for employees by name, then Oracle will be doing

a full table scan for each name lookup The response time won't be too good, and your users won't be too happy

Creating an index on the employee name field would improve your results a great deal

Don't overlook the possibility of creating a multicolumn index, even if you don't use all the index columns in your query's WHERE clause Suppose that you frequently execute the following query, which searches for an employee by name and displays that employee's current billing rate:

SELECT employee_name, employee_billing_rate

FROM employee

WHERE employee_name = :emp_name;

If you index the employee table by name, Oracle will look up the name in the index, get the ROWID, read the correct employee record into memory, and then return the billing rate However, it takes an extra read from disk to fetch the employee record into memory You can eliminate that extra I/O by creating an index such as this one:

CREATE INDEX employee_by_name ON employee

(employee_name, employee_billing_rate) ;

Since the index now contains the billing rate column, Oracle does not need to read the actual employee record in order

to retrieve it Oracle is smart enough to recognize that all the columns needed to satisfy the query are in the index, and it will take advantage of that fact

Rewriting the Query.

Sometimes you can restate a query, still get the results that you want, and have it run much more efficiently Consider the following query, which lists all employees who have ever charged time to project 1001 or 1002:

SELECT DISTINCT employee.employee_id, employee.employee_name

FROM employee, project_hours

WHERE employee.employee_id = project_hours.employee_id

AND project_hours.project_id in (1001,1002);

The execution plan for this query looks like this:

0 SELECT STATEMENT Cost = 8

1 SORT UNIQUE

2 HASH JOIN

3 TABLE ACCESS FULL EMPLOYEE

4 TABLE ACCESS FULL PROJECT_HOURS

The query could be restated using an EXISTS predicate instead of joining the employee and project_hours tables

together Here's how that would look:

SELECT employee_id, employee_name

FROM employee

WHERE exist (SELECT *

FROM project _hours

WHERE project_hours.project_id = 1001

AND project_hours.employee_id = employee.employee_id)

or exist (SELECT *

FROM project_hours

WHERE project_hours.project_id = 1002

AND project_hours.employee_id = employee.employee_id);

The execution plan for this version of the query looks like this:

0 SELECT STATEMENT Cost = 1

1 FILTER

2 TABLE ACCESS FULL EMPLOYEE

3 INDEX RANGE SCAN PROJECT_HOURS_PK

4 INDEX RANGE SCAN PROJECT_HOURS_PK

The second version of the query has a cost of 1, as opposed to a cost of 8 for the first Thus, the second version of the query is the one to use The higher cost of the first query probably comes from the SORT, which is necessary to

eliminate duplicate rows, and from the full table scan on the PROJECT_HOURS table

Using Hints

Rather than allowing Oracle to have total control over how a query is executed, you can provide specific directions to

the optimizer through the use of hints A hint, in Oracle, is an optimizer directive embedded in a SQL statement in the

form of a comment Here is a query with an optimizer hint telling Oracle to do a full table scan:

SELECT /*+ full (employee) */

employee_id, employee_name, employee_billing_rate

FROM employee

WHERE employee_name = Jenny Gennick;

The hint in this case is full(employee), which tells Oracle to do a full table scan of the employee table Oracle will honor this hint and perform a full table scan, even if there happens to be an index on the employee name field

Syntax for a hint

A hint applies to a single SQL statement, and hints may be specified only for SELECT, INSERT, UPDATE, and

DELETE statements The hint takes the form of a specially formatted comment, and must appear immediately following the keyword that begins the statement The hint takes the form:

keyword /*+ [hint\comment] */

where:

keyword

Is the keyword that begins the statement This will be one of the keywords SELECT, UPDATE, or DELETE Hints do not apply to any other type of statement The hint must immediately follow the keyword that begins the statement

hint

The hint itself, sometimes with one or more arguments enclosed in parenthesis Tables 8-2 through 8-7 provide a complete list of possible hints Hints are not case-sensitive A single comment may contain more than one hint, as long as the hints are separated by at least one space

comment

A user-specified comment Oracle allows you to intersperse comments with your hints

Here are some examples of how hints may be specified:

SELECT /*+ full(employee) */ employee_id, employee_name

FROM employee

WHERE employee_billing_rate > 100;

SELECT /*+ full(e) do a full table scan on the employee table, because

most employees do have billing rates > 100 */

employee_id, employee_name

FROM employee e

WHERE employee_billing_rate > 100;

Note the second example The table name is employee, but an alias of e has been specified The hint for the table uses the same alias, and is specified as full(e) Whenever an alias is used, you must also use the alias name in any hints for the table

If you want to supply multiple hints for a statement, they must all appear in the same comment, for example:

SELECT /*+ full(employee) first_rows */

employee_id, employee_name

FROM employee

WHERE employee_billing_rate > 100;

When subqueries are used, they are allowed to have their own hints The hint for a subquery follows immediately after the keyword that starts the query, for example:

SELECT /*+ first_rows */ employee_id, employee_name

FROM employee

WHERE exists (SELECT /*+ full(project_hours) */*

FROM project_hours

WHERE project_hours.project_id = 1001

AND project_hours.employee_id = employee.employee_id);

When using hints, be very careful to get the syntax exactly right Because hints are embedded in the statements as comments, Oracle can't

do any syntax checking Oracle treats any incorrectly specified hint as a comment In addition, you should always do an EXPLAIN PLAN after you code your hints, just to be sure that the optimizer is really doing what you think you told it to do.

Oracle hints can loosely be divided into the following categories:

Optimizer goal hints

Access method hints

Join order hints

Join operation hints

Parallel execution hints

Other hints

The next few sections describe the hints available in each category.

Optimizer goal hints

Optimization goal hints allow you to influence the optimizer's overall goal when formulating an execution plan You may, for example, specify that you want the plan optimized to return the first record as quickly as possible Table 8-2 gives a list of these hints.

Table 8-2 Optimization Goal Hints

Table 8-2 Optimization Goal Hints

Hint Description

ALL_ROWS Tells the optimizer to produce an execution plan that minimizes resource consumption.

FIRST_ROWS Tells the optimizer to produce an execution plan with the goal of getting to the first row as quickly as

possible.

CHOOSE Allows the optimizer to choose between the rule-based mode and the cost-based mode If statistics are

present for any tables in the query, the cost-based approach will be taken.

RULE Forces the optimizer to use a rule-based approach for the statement.

You should avoid the RULE hint if at all possible That hint causes the rule-based optimizer to be used The rule-based optimizer uses a fixed set of rules when determining the execution plan for a statement, and does not attempt to factor in the ultimate cost of executing that plan The cost-based optimizer, on the other hand, will base its decision on the estimated I/O and CPU overhead required by various alternative plans While Oracle still supports the rule-based optimizer, it hasn't been enhanced in years, won't be enhanced in the future, and may be

supported at some point Oracle is putting their development effort into the costbased optimizer.

Access method hints

Access method hints allow you to control the way data is accessed For example, you can tell Oracle to do a full table scan, or to use an index

when accessing a table You can name the specific index to be used Table 8-3 provides a list of these hints.

Table 8-3 Access Method Hints

FULL(table_name) Requests a full table scan of the specified table, regardless of any indexes that may exist.

ROWID(table_name) Tells Oracle to perform a scan of the specified table based on ROWIDs.

CLUSTER(table_name) Tells Oracle to do a cluster scan of the specified table This hint is ignored if the table is not clustered.

HASH(table_name) Tells Oracle to do a hash scan of the specified table This hint is ignored if the table is not clustered.

HASH_AJ(table_name) Tells Oracle to do a hash anti-join of the specified table.

INDEX(table_name) Tells Oracle to access the specified table via and index scan Optionally, you may specify the index to

use; otherwise, Oracle chooses the index to use; otherwise, Oracle chooses the index You may also specify a list to indexes to choose from, and Oracle will choose from that list.

INDEX_ASC (table_name [index_name]) Similar to the INDEX hint, but tells Oracle to scan the index in ascending order.

INDEX_COMBINE(table_name [index_name]) Tells Oracle to use some combination of two indexes You may specify the indexes to choose from, or let

Oracle make the choice.

INDEX_DESC(table_name[index_name]) Similat to INDEX_ASC, but forces Oracle to scan the index in descending order.

INDEX_FFS(table_name[index_name]) Tells Oracle to do a fast full index scan.

MERGE_AJ(table_name) Turns a NOT IN subquery into a merge anti-join.

AND_EQUAL(table_name index_name index name) Tells Oracle to scan two or more indexes and merge the results You must specify at least two index

names.

USE_CONCAT Turns a query with OR conditions into two or more queries unioned together with a UNION ALL.

All access method hints take at least a table name as an argument That's because you may want to specify different access methods for

different tables in your query The FULL hint, for example, takes one table name as an argument.

/*+ FULL(employee) */

Some of the access method hints are index-related, and allow you to specify one or more indexes to be used In many cases, as with the INDEX hint, you have the choice of specifying an index name or not The following hint, for

example, tells Oracle you want to do an index scan on the employee table, but it's up to Oracle to pick the index:

/*+ INDEX(employee) */

This is useful if you think Oracle will make the correct choice, or if you don't want to hardcode an index name into the hint You have the option, however, of specifying the exact index to use Here's an example:

/*+ INDEX (empolyee employee_by_name) */

You may even specify a list of indexes, and Oracle will choose from the indexes in that list If, for example, you had seven indexes on the employee table, but you believed that only two would be at all useful for the query in question, you could specify a hint like this:

/*+ INDEX(employee employee_by_name, employee_by_billing_rate) */

This tells Oracle that you want to use an index scan to access the employee table, and that you want to use either the name index or the billing rate index

The AND_EQUAL hint is special in that it requires at least two indexes to be specified That's because this hint causes Oracle to merge the results of two index scans together You can't do that unless you have two indexes to scan

If your choice conflicts with other hints, if it cannot be implemented, or if the indexes you specify do not exist, Oracle will simply ignore the hint altogether Take a look at the following query:

SELECT /*+ USE_CONCAT */

employee_id, employee_name

FROM employee

WHERE employee_name = Jeff Gennick;

The USE_CONCAT hint makes no sense here because the query does not contain an OR condition You can't break this

up into two queries and then UNION the results together, so Oracle will ignore the hint A bad hint will be honored, however, whenever it is possible to implement The following query contains a hint to do an index scan on the primary key index for the employee table:

SELECT /*+ INDEX(employee employee_pk) */

employee_name

FROM employee

WHERE employee_name = Jeff Gennick;

page_304

The primary key for employee is the employee_id field An index on employee name does exist The query seeks one record based on the

employee name Even though it makes perfect sense to look up the name in the name index, Oracle will honor the request to use the primary

key index Here is the execution plan for this statement:

0 SELECT STATEMENT Cost =826

1 TABLE ACCESS BY INDEX ROWID EMPLOYEE

2 INDEX FULL SCAN EMPLOYEE_PK

Oracle is going to read every entry in the primary key index, retrieve the associated row from the employee table, and check the name to see if it

has a match This is worse than a full table scan! Oracle does this because the hint requested it, and because it physically can be done, so be

careful what you ask for, and check the results Change the index name used in the query to employee_by_name, and the execution plan looks

like this:

0 SELECT STATEMENT Cost = 1

1 INDEX RANGE SCAN EMPLOYEE_BY_NAME

You can see that the relative cost of using the name index is much, much less than that of using the primary key index Be careful with these hints.

Join order hints

Join order hints allow you to exercise some control over the order in which Oracle joins tables There are only three of them, and they are listed

in Table 8-4.

Table 8-4 Join Order Hints

ORDERED Tells Oracle to join tables left to right, in the same order in which they are listed in the FROM clause.

STAR Tells Oracle to use a star query execution plan, if at all possible This can only work if there are at least three

tables being joined, and the largest table has a concatenated index on columns that reference the two smaller tables The two smaller tables are joined first, and then a nested-loop join is used to retrieve the required rows from the largest table.

STAR_TRANSFORMATION Tells Oracle to transform the query into a star query, if possible, and then use the best plan for that query.

Join operation hints

Join operation hints allow you to control the manner in which two tables are joined Oracle uses three basic methods whenever two tables are

joined: the merge join, the nested loops join, and the hash join.

A merge join is done by sorting the rows from each table by the join columns Once the two rowsets have been sorted, Oracle reads through both

and joins any matching rows together A merge join often uses fewer resources than the other options, but you have to wait for all the records to

be sorted before you get the first one back You also have to have enough memory and temporary disk space to handle the sort.

The method used for a nested loops join corresponds to the mental image most people have in mind when they think of joining tables Oracle

picks one table as the driving table, and reads through that table row by row For each row read from the driving table, Oracle looks up the

corresponding rows in the secondary table and joins them together Because no sort is involved, a nested loops join will usually get you the

first record back more quickly than a merge join For the same reason, a nested loops join also does not require large amounts of disk space

and memory However a nested loops join may result in a considerably greater number of disk reads than a merge join.

A hash join is similar to a merge join, but a sort is not required A hash table is built in memory to allow quick access to the rows from one of

the tables to be joined Then rows are read from the other table As each row is read from the second table, the hash function is applied to the

join columns, and the result is used to find the corresponding rows from the first table.

Aside from the hints used to specify the join method, there are a few other hints lumped into the join operation category Table 8-5 lists all the

join operation hints.

Table 8-5 Join Operation Hints

USE_NL(table_name) Tells Oracle to use a nested loop when joining this table The table specified by this hint will be the one accessed

by the innermost loop The other table will be the driving table.

USE_MERGE(table_name) Tells Oracle to use the sort merge method when joining this table.

USE_HASH(table_name) Tells Oracle to use a hash join for the specified table.

NO_MERGE This is not the opposite of USE_MERGE The NO_MERGE hint applies to queries that contain joins on one or

more views It prevents Oracle from merging the query from a view into the main query.

DRIVING_SITE(table_name) This hint applies when you are executing a distributed join, one that joins tables from two or more databases

Without a hint, Oracle will choose which database actually collects the tables and does the join By using the hint, you are telling Oracle that you want the join performed by the database containing the specified table.

Parallel execution hints

The hints shown in Table 8-6 allow you to influence the way Oracle executes a query in a parallel processing environment In an environment with

a single CPU, parallel processing is not possible, and these hints will be ignored.

Table 8-6 Parallel Execution Hints

PARALLEL(table_name [, degree[, num_instances]]) Tells Oracle to access data from the indicated table in a parallel processing mode You

can optionally specify both the degree of parallelism to use and the number of instances that will be involved The keyword DEFAULT may be used for both arguments, in

which case Oracle decides the values based on parameters in the INIT.ORA file and the

table definition Using the PARALLEL hint in an INSERT statement automatically turns APPEND mode on See the APPEND and NO_APPEND hints.

NO_PARALLEL(table_name) Tells Oracle not to access the specified table in parallel.

APPEND Applies only to INSERT statements It tells Oracle not to attempt to reuse any freespace

that may be available in any extents currently allocated to the table.

NOAPPEND This is the opposite of APPEND, and tells Oracle to use any freespace in extents

currently allocated to the table This hint exists because APPEND becames the default behavior whenever a PARALLEL hint is used in an INSERT statement.

PARALLEL_INDEX(table_name,index_name [,degree [,num_instances]]) Tells Oracle to access data from the indicated table by scanning the specified index in a

parallel processing mode The index must be a partitioned index You can optionally specify both the degree of parallelism to use and the number of instances that will be involved The keyword DEFAULT may be used for both arguments, in which case

Oracle decides the values based on parameters in the INIT.ORA file and the table

definition.

Other hints

There are a few hints that don't fit neatly into one of the other categories These are listed in Table 8-7.

Table 8-7 Other Hints

Hint Description

CACHE(table_name) Applies only when a full table scan is being performed on the specified table It tells Oracle to place blocks for that table at the

most recently used end of the buffer cache, so they will remain in memory as long as possible This can be useful for small lookup tables that you expect to access repeatedly.