Which of the below would be the best way to create an Index, so that my query gives faster results?
My query is:
select emp_name,emp_last_name, salary
from employees_table
where salary <=2000;
1. create index emp_index on employees_table (salary);
2. create index emp_index on employees_table (emp_name,emp_last_name,salary);
In principle create index emp_index on employees_table (emp_name,emp_last_name,salary); would be the better one, because for your query Oracle has to read only the index but not any table data.
However, this means you create a separate index for each particular query which is certainly an overkill.
For 2 index Oracle may will be select table access full, if disctinct values in firsts columns of index to much, Oracle give you Table access full, if distinct values in first columns of index not much then you give index skip scan.
For first index you will see “index range scan” + “table access by rowid”....
but for both index’s - if sql returns >5-7% rows then Oracle maybe want to use “Table access full”...
All depends on your data in Table and count return rows
Related
I have table A(id,name,code)
I have sql statement:
Select * from A where upper(code || name) like upper('%<search text>%');
How to create an index so that the following statement has an index?
Question for two option: table partitioned, and table not partitioned
Thanks & BR
Do you have a performance issue or is this just a hypothetical question?
An index is unlikely to help with this example: a full table scan will probably be the quickest solution. Why? Your table has 3 columns. The best index would be one that avoided looking in the table at all e.g.
create index ai on a (code, name, id);
But that needs to contain all the same data as the table plus a ROWID for each table row - so it is going to be bigger than the table and take longer to scan. You could try putting the columns in the index with the least selective first and using compression:
create index ai on a (code, name, id) compress;
Now the index may be smaller than the table - it depends on how selective the code and name columns are. If it is small enough, the optimizer might decide to use it instead of the table. It still contains all the IDs and ROWIDs so the reduction in size probably won't be dramatic. In the test case I set up the compressed index is about half the size of the table, yet Explain Plan shows the query has a higher cost if I use a hint to force it to use the index - maybe due to overheads of compression, I don't know.
You could look into Oracle Text and the CONTAINS expression - but then you would be writing a different query, not using LIKE.
I have a large table of 7 column in Oracle 11G. Total size of the table is more than 3GB and total row in this table is 1876823. Query we are using
select doc_mstr_id from index_mstr where page_con1 like('%sachin%') it is taking almost a minute. please help me to optimize the query as well as proper indexing for this table. Please let me know if partitioned is required for this table.
Below are the column description
INDEX_MSTR_ID NUMBER
DOC_MSTR_ID NUMBER
PAGE_NO NUMBER
PAGE_PART NUMBER
PAGE_CON1 VARCHAR2(4000)
FILE_MODIFIED_DATE DATE
CREATED_DATE DATE
This query is always going to result in a full table scan. Your only filter cannot use a B-TREE index, due to the leading wildcard:
where page_con1 like('%sachin%')
If you want to do lots of queries of this nature you need to build a Text index on that column. From its datatype page_con1 appears to hold text fragments rather than full documents so you should use a CTXCAT index. This type of index has the advantage of being transactional, rather than requiring background maintenance. Find out more.
Your query would then look like this:
select doc_mstr_id from index_mstr
WHERE CATSEARCH(page_con1, 'sachin') > 0;
I have a massive table in which I can't do any more partitioning or sub-partitioning, nor am allowed to do any alter. I want to query its records by batches, and thought a good way would be using the last two digits from the account numbers (wouldn't have any other field splitting records as evenly).
I guess I'd need to at least index that somehow (remember I can't alter table to add a virtual column either).
Is there any kind of index to be used in such situation?
I am using Oracle 11gR2
You can use function based index:
create index two_digits_idx on table_name (substr(account_number, -2));
This index will work only in queries like that:
select ...
from table_name t ...
where substr(account_number, -2) = '25' -- or any other two digits
For using index, you need to use in a query the same expression like in an index.
I have a table that I expect to get 7 million records a month on a pretty wide table. A small portion of these records are expected to be flagged as "problem" records.
What is the best way to implement the table to locate these records in an efficient way?
I'm new to Oracle, but is a materialized view an valid option? Are there such things in Oracle such as indexed views or is this potentially really the same thing?
Most of the reporting is by month, so partitioning by month seems like an option, but a "problem" record may be lingering for several months theorectically. Otherwise, the reporting shuold be mostly for the current month. Would you expect that querying across all month partitions to locate any problem record would cause significant performance issues compared to usinga single table?
Your general thoughts of where to start would be appreciated. I realize I need to read up and I'll do that but I wanted to get the community thought first to make sure I read the right stuff.
One more thought: The primary key is a GUID varchar2(36). In order of magnitude, how much of a performance hit would you expect this to be relative to using a NUMBER data type PK? This worries me but it is out of my control.
It depends what you mean by "flagged", but it sounds to me like you would benefit from a simple index, function based index, or an indexed virtual column.
In all cases you should be careful to ensure that all the index columns are NULL for rows that do not need to be flagged. This way your index will contain only the rows that are flagged (Oracle does not - by default - index rows in B-Tree indexes where all index column values are NULL).
Your primary key being a VARCHAR2 GUID should make no difference, at least with regards to the specific flagging of rows in this question, indexes will point to rows via Oracle internal ROWIDs.
Indexes support partitioning, so if your data is already partitioned, your index could be set to match.
Simple column index method
If you can dictate how the flagging works, or the column already exists, then I would simply add an index to it like so:
CREATE INDEX my_table_problems_idx ON my_table (problem_flag)
/
Function-based index method
If the data model is fixed / there is no flag column, then you can create a function-based index assuming that you have all the information you need in the target table. For example:
CREATE INDEX my_table_problems_fnidx ON my_table (
CASE
WHEN amount > 100 THEN 'Y'
ELSE NULL
END
)
/
Now if you use the same logic in your SELECT statement, you should find that it uses the index to efficiently match rows.
SELECT *
FROM my_table
WHERE CASE
WHEN amount > 100 THEN 'Y'
ELSE NULL
END IS NOT NULL
/
This is a bit clunky though, and it requires you to use the same logic in queries as the index definition. Not great. You could use a view to mask this, but you're still duplicating logic in at least two places.
Indexed virtual column
In my opinion, this is the best way to do it if you are computing the value dynamically (available from 11g onwards):
ALTER TABLE my_table
ADD virtual_problem_flag VARCHAR2(1) AS (
CASE
WHEN amount > 100 THEN 'Y'
ELSE NULL
END
)
/
CREATE INDEX my_table_problems_idx ON my_table (virtual_problem_flag)
/
Now you can just query the virtual column as if it were a real column, i.e.
SELECT *
FROM my_table
WHERE virtual_problem_flag = 'Y'
/
This will use the index and puts the function-based logic into a single place.
Create a new table with just the pks of the problem rows.
We have Oracle 10g and we need to query 1 table (no joins) and filter out rows where 1 of the columns is null. When we do this - WHERE OurColumn IS NOT NULL - we get a full table scan on a very large table - BAD BAD BAD. The column has an index on it but it gets ignored in this instance. Are there any solutions to this?
Thanks
The optimizer thinks that the full table scan will be better.
If there are just a few NULL rows, the optimizer is right.
If you are absolutely sure that the index access will be faster (that is, you have more than 75% rows with col1 IS NULL), then hint your query:
SELECT /*+ INDEX (t index_name_on_col1) */
*
FROM mytable t
WHERE col1 IS NOT NULL
Why 75%?
Because using INDEX SCAN to retrieve values not covered by the index implies a hidden join on ROWID, which costs about 4 times as much as table scan.
If the index range includes more than 25% of rows, the table scan is usually faster.
As mentioned by Tony Andrews, clustering factor is more accurate method to measure this value, but 25% is still a good rule of thumb.
The optimiser will make its decision based on the relative cost of the full table scan and using the index. This mainly comes down to how many blocks will have to be read to satisfy the query. The 25%/75% rule of thumb mentioned in another answer is simplistic: in some cases a full table scan will make sense even to get 1% of the rows - i.e. if those rows happen to be spread around many blocks.
For example, consider this table:
SQL> create table t1 as select object_id, object_name from all_objects;
Table created.
SQL> alter table t1 modify object_id null;
Table altered.
SQL> update t1 set object_id = null
2 where mod(object_id,100) != 0
3 /
84558 rows updated.
SQL> analyze table t1 compute statistics;
Table analyzed.
SQL> select count(*) from t1 where object_id is not null;
COUNT(*)
----------
861
As you can see, only approximately 1% of the rows in T1 have a non-null object_id. But due to the way I built the table, these 861 rows will be spread more or less evenly around the table. Therefore, the query:
select * from t1 where object_id is not null;
is likely to visit almost every block in T1 to get data, even if the optimiser used the index. It makes sense then to dispense with the index and go for a full table scan!
A key statistic to help identify this situation is the index clustering factor:
SQL> select clustering_factor from user_indexes where index_name='T1_IDX';
CLUSTERING_FACTOR
-----------------
460
This value 460 is quite high (compared to the 861 rows in the index), and suggests that a full table scan will be used. See this DBAZine article on clustering factors.
If you are doing a select *, then it would make sense to do a table scan rather than using the index. If you know which columns you are interested in, you could create a covered index with those colums plus the one you are applying the IS NOT NULL condition.
It can depend on the type of index you have on the table.
Most B-tree indexes do not store null entries. Bitmap indexes do store null entries.
So, if you have:
select * from mytable
where mycolumn is null
and you have a standard B-tree index on mycolumn, then the query can't use the index as the "null" isn't in the index.
(If the index is against multiple columns, and one of the indexed columns is not null then there will be an entry in the index.)
Create an index on that column.
To make sure the index is used, it should be on the index and other columns in the where.
ocdecio answered:
If you are doing a select *, then it would make sense to do a table scan rather than using the index.
That's not strictly true; an index will be used if there is an index that fits your where clause, and the query optimizer decides using that index would be faster than doing a table scan. If there is no index, or no suitable index, only then must a table scan be done.
It's also worth checking whether Oracle's statistics on the table are up to date. It may not know that a full table scan will be slower.
Oracle database don't index null values at all in regular (b-tree) indexes, so it can't use it nor you can't force oracle database to use it.
BR
Using hints should be done only as a work around rather than a solution.
As mentioned in other answers, the null value is not available in B-TREE indexes.
Since you know that you have mostly null values in this column, would you be able to replace the null value by a range for instance.
That really depends on your column and the nature of your data but typically, if your column is a date type for instance:
where mydatecolumn is not null
Can be translated in a rule saying: I want all rows which have a date.
Then you can most definitely do this:
where mydatecolumn <=sysdate (in oracle)
This will return all rows with a date and ommit null values while taking advantage of the index on that column without using any hints.
See http://www.oracloid.com/2006/05/using-index-for-is-null/
If your index is on one single field, it will NOT be used. Try to add a dummy field or a constant in the index:
create index tind on t(field_to_index, 1);