Explain merge join in oracle SQL .

-

 

Introduction:

Merge join in Oracle SQL PLSQL topic is a method which is used to join the two sorted datasets efficiently. It is effective when both the input tables are large. It is also called as sort merge join. It sorts both the input sets i.e. tables or subqueries which is already sorted on the join key columns if they are not already sorted.

Here, input datasets means that tables or subqueries .In the other words, it is the type of join operation which is used by the oracle optimizer which combine the rows from the two sorted datasets which is based on the join condition by using  an equi join(=).Sorted data is merged like the zipper.In this,memory usuage is medium.

How it works?

It checks if the data from the two tables which is already sorted on the join keys. If not, it will sort them.

Both the input tables or the result sets are sorted on the join key.

Matching rows which are joined and returned.

When it is used?

Both the inputs of the tables are large and can be sorted efficiently.

If there is an index or order by which already sorts the data.

When the hash join is not optimal and the nested loop join would be slow due to the large volumes.

When the join is an equi join.

Both the tables are large.

Sorting is cheap or already done due to indexes and previous operations.

It is used when the join column are sorted by the range indexes.

select e.empno,d.dname

from emp e

JOIN dept d

ON e.deptno=d.deptno;


If emp and dept are both sorted by the deptno, oracle may use the merge join.

Data is retrieved in the sorted manner with the help of indexes or no sorting is needed & merge the two sorted datasets efficiently.

Summary:

It minimizes the repeated scans which is by leveraging the sorted order.

When indexes or pre-sorted data will reduce the sorting overhead.

Optimizer automatically select when the cost estimates Favour over an alternative.

Advantages:

It is efficient for large and sorted datasets.

It works for equality and range conditions.

After sorting linear merge phase.

Disadvantages:

Memory or disk is required for sorting.

It is not ideal for the small tables.

Sorting the unsorted data includes overhead.



Comments

Post a Comment

Popular posts from this blog

Difference between union and union all in oracle sql plsql

-
Image
"This is one of the most important questions that interviewer might be asking in an interview.Infyosis,Linked group,Kyndryl,Vodafone Idea, IBM and Mphasis etc." INTRODUCTION: It is said to be set operators which is used to combine, add or concatenate two or more result sets. It is used in the multiple select statements where to combine to produce the final desired results. In other words, multiple select statements of similar column name in same order and having same data type to produce the unified result. It merges the content of the two structurally compatible organized tables into single table.  In MIS datalake,when you are adding the modules of GST reconciliation which is to be used in micros in Excel or VB script, union and union all set operator is preferred for eliminating duplicate records or keeping duplicate records as per business requirement. There are several alternatives like bulk collect,join,set operators like intersect & minus for combining the data from...
Read more

Explain oracle pl sql datatypes interview question and answer for fresher and experienced.

-
Image
 Oracle PLSQL Data Types : PLSQL constant,value,variable,parameter,function which return the value are having the data type. It will determine the storage space information occupied and occurred in the memory or storage format and range constraints for valid values and operations as per specified size limit. It will determine how the data will be going to stored, handled and treated by the oracle engine during the period of processing and storage. It is used in the PLSQL blocks. It allows you to define your sub-types. PLSQL codes are embedded into the java program. Subtypes are used to make the data types compatible with data types used in the PLSQL program while embedding the PLSQL code into the java program. Subtype will be compatible with ANSI/ISO & IBM. It provides the pre-defined data types. Pre-defined data types are categorized into the four types: Composite LOB Reference Scalar Scalar Datatypes: It stores the values without the internal components. It holds the single ...
Read more

Difference between full join and cross join-oracle sql plsql

-
Image
 Difference between full join and cross join in sql:- They behave differently to combine the rows from the two tables. FULL OUTER JOIN: It is also called as the full join .It will return all the rows when there is either match in the left table or right table. If there is no match, it will return NULL for the rows of the columns which is lacking the matching rows. Syntax: select c.*,d.*  from tableC c FULL OUTER JOIN tableD d on c.id=d.id; select e.ename,d.dname from emp e FULL OUTER JOIN dept d ON e.deptno=d.deptno; When you want to retain the unmatched rows i.e operations which is not present in the emp table and having deptno as 40.So,ename is appeared as NULL. CROSS JOIN: It returns the cartesian product of both the tables i.e. Every row of the table A is combined with every row of the table B.ON condition is not used due to not match rows. Syntax: select c.*.d.* from tableC c CROSS JOIN tableD d; select e.ename,d.dname from emp e CROSS JOIN dept d; ENAME ...
Read more