Sorting and Joining

10Copyright © 2005, Oracle. All rights reserved.

Sorting and Joining

10-2 Copyright © 2005, Oracle. All rights reserved.

Objectives

After completing this lesson, you should be able to do the following:• Optimize sort performance• Describe different join techniques• Explain join optimization• Find optimal join execution plans


Sort Operations

• SORT UNIQUE• SORT AGGREGATE• SORT GROUP BY • SORT JOIN• SORT ORDER BY


Tuning Sort Performance

• Sorting large sets can be expensive, so you should tune sort parameters.

• Note that that DISTINCT, GROUP BY, and most set operators cause implicit sorts.

• Minimize sorting by one of the following:– Try to avoid DISTINCT and GROUP BY.– Use UNION ALL instead of UNION.– Enable index access to avoid sorting.


Top-N SQL

SQL> SELECT * 2 FROM (SELECT prod_id 3 , prod_name 4 , prod_list_price 5 , prod_min_price 6 FROM products 7 ORDER BY prod_list_price DESC) 8 WHERE ROWNUM <= 5;


Join Terminology

• Join statement• Join predicate, nonjoin predicate• Single-row predicate

SQL> SELECT c.cust_last_name,c.cust_first_name, 2 co.country_id, co.country_name 3 FROM customers c JOIN countries co 4 ON (c.country_id = co.country_id) 5 AND ( co.country_id = 'JP' 6 OR c.cust_id = 205);

Join PredicateNon join Predicate

Single row Predicate


Join Terminology• Natural join

• Join with nonequal predicate

SQL> SELECT c.cust_last_name, co.country_name 2 FROM customers c NATURAL JOIN countries co;

SQL> SELECT s.amount_sold, p.promo_name 2 FROM sales s JOIN promotions p 3 ON( s.time_id 4 BETWEEN p.promo_begin_date 5 AND p.promo_end_date );

• Crossjoin SQL> SELECT * 2 FROM customers c CROSS JOIN countries co;


Join Operations

• A join operation combines the output from two row sources and returns one resulting row source.

• Join operation types include the following:– Nested loops join– Sort and merge join– Hash join


Nested Loops Joins

• One of the two tables is defined as the outer table (or the driving table).

• The other table is called the inner table.• For each row in the outer table, all matching rows

in the inner table are retrieved.

For each row in the outer table

For each row in the inner table

Check for a match


Nested Loops Join Plan

Nested loops

Table access(Outer/driving table)

Table access(Inner table)

1

2 3


When Are Nested Loops Joins Used?

Nested loops joins are used when:• Joining a few rows that have a good driving

condition• Order of tables is important• USE_NL(table1 table2) hint is used


Hash Joins

A hash join is executed as follows:• Both tables are split into as many partitions as

required, using a full table scan.• For each partition pair, a hash table is built in

memory on the smallest partition.• The other partition is used to probe the hash table.


Hash Join Plan

Hash join

Table access Table access

1

2 3


When Are Hash Joins Used?

Hash joins are used if either of the following conditions is true:• A large amount of data needs to be joined. • A large fraction of the table needs to be joined.

Use the USE_HASH hint, and use correct values for the following initialization parameters:• HASH_AREA_SIZE • HASH_JOIN_ENABLED


Sort and Merge Joins

A sort and merge join is executed as follows:1. The rows from each row source are sorted

on the join predicate columns.2. The two sorted row sources are then merged

and returned as the resulting row source.


Sort and Merge Join Plan

2 3

Merge

Sort Sort

Table access Table access

1

4 5


When Are Sort and Merge Joins Used?

Sort and merge joins can be used if any of the following conditions are true:• Join condition is not an equijoin.• OPTIMIZER_MODE is set to RULE.• HASH_JOIN_ENABLED is false. • Sorts are required for other operations.• Optimizer has considered the costs based on the

values of:– HASH_AREA_SIZE– SORT_AREA_SIZE


Joining Multiple Tables

You can join only two row sources at a time. Joins with more than two tables are executed as follows:1. Two tables are joined, resulting in a row source.2. The next table is joined with the row source that

results from step 1.3. Repeat step 2 until all tables are joined.


SQL:1999 Outer Joins

• Without using plus (+) sign .• The keyword OUTER JOIN is used instead.

SQL> SELECT s.time_id, t.time_id 2 FROM sales s 3 RIGHT OUTER JOIN times t 4 ON (s.time_id = t.time_id);


Oracle Proprietary Outer Joins

• Join predicates with a plus (+) sign• Nonjoin predicates with a plus (+) sign• Predicates without a plus (+) sign disable outer

joins.

SQL> SELECT s.time_id, t.time_id 2 FROM sales s, times t 3 WHERE s.time_id (+) = t.time_id;


Full Outer Joins

• A full outer join acts like a combination of the left and right outer joins.

• In addition to the inner join, rows in both tables that have not been returned in the result of the inner join are preserved and extended with nulls.

• This syntax has been introduced in Oracle9i Database.

SQL> SELECT c.cust_id, c.cust_last_name 2 , co.country_name 3 FROM customers c 4 FULL OUTER JOIN countries co 5 ON (c.country_id = co.country_id);


Execution of Outer Joins

Indexes can be used for outer join predicates.

SQL> SELECT c.cust_id, co.country_name 2 FROM customers c 3 LEFT OUTER JOIN countries co 4 ON (c.country_id = co.country_id) 5 AND co.country_id = 'IT';


The Optimizer and Joins

The optimizer determines the following (in sequence):1. Order in which to join the tables2. Best join operation to apply for each join3. Access path for each row source


Join Order Rules

Rule 2

For outer joins, the table with the outer joined table must come after the other table in the join order for processing the join.

Rule 1

A single-row predicate forces its row source to be placed first in the join order.


Join Optimization

• As a first step, a list of possible join orders is generated.

• This potentially results in the following:

• Parse time grows exponentially when adding tables to a join.

Number of Tables Join Orders---------------- -----------2 2! = 23 3! = 64 4! = 24


Join Optimization

1. A set of possible execution plans is generated, limited by the following initialization parameter:OPTIMIZER_MAX_PERMUTATIONS

2. The optimizer then estimates the cost of each plan and chooses the one with the lowest cost.


Estimating Join Costs

• Nested loops join:Cost read(A) + card(A) * read(B)– OPTIMIZER_INDEX_CACHING parameter

• Sort and merge join:Cost read(A) + sort(A) + read(B) + sort(B)


Star Joins

SALES

PRODUCTS

CHANNELS PROMOTIONS TIMES

CUSTOMERS


Hints to Facilitate Joins

Join order• ORDERED• LEADING• STAR

Join operations• USE_HASH,USE_MERGE,

USE_NL• LEADING• HASH_AJ, MERGE_AJ,

NL_AJ• HASH_SJ, MERGE_SJ,

NL_SJ


Other Join Hints

SQL> select /*+ STAR_TRANSFORMATION */... 2 from T1, T2, ... 3 where ...

SQL> select /*+ DRIVING_SITE(T1) */... 2 from T1, T2, ... 3 where ...


Subqueries and Joins

• Subqueries, like joins, are statements that reference multiple tables.

• Subquery types:– Noncorrelated subqueries– Correlated subqueries– NOT IN subqueries (anti-joins)– EXISTS subqueries (semi-joins)


Initialization Parameters ThatInfluence Joins

• Hash join parameters:– HASH_JOIN_ENABLED– HASH_AREA_SIZE

• Sort parameters:– SORT_AREA_SIZE– SORT_AREA_RETAINED_SIZE


Throwaway of Rows

• Rows that are retrieved but not used• Identify throwaway by comparing the number of

rows of the following:– Join operation– Input row sources


Minimize Throwaway of Rows

• Compare available indexes with join and nonjoin predicates.

• Consider hinting for a nested loop instead of sort and merge operations.


Minimize Processing

• Changing nested loops joins into sort and merge joins can do the following:– Remove index access– Add sort overhead

• Theoretically, hash joins are the most efficient join operation.

• Cluster joins need less I/O than the corresponding nested loops join.


Summary

In this lesson, you should have learned how to:• Use the Top-N SQL feature• Describe available join operations• Optimize join performance against different

requirements• Influence the join order• Discover that tuning joins is more complicated

than tuning single table statements

Documents

Sorting and Joining