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.
Related
We have a table that doesn't have much data. The table has 3 partitions and we are deleting data in one partition only.
delete from table AB partition(A) where id=value;
here id has an index also but still delete is slow.
The datatype of id is varchar2 and the value is number.
Please help me to understand why the delete statement is slow.
I don't think the index has much use in this case. It has to evaluate every single row in the partition to see if it matches id=value. Typically this will be a full table scan and no index will be used. It totally depends on the number of rows in the partition how long it will take. But maybe i did not understand the question properly. I presumed "value" is a column in the same table, like ID.
Consider this Oracle docs about indexes, this about speed of insert and this question on StackOverflow lead me to conclusion that:
Indexes helps us locate information faster
Primary and Unique Keys are indexed automatically
Inserting with indexes can cause worse performance
However every time indexes are discussed there are only SELECT operations shown as examples.
My question is: are indexes used in INSERT and UPDATE operations? When and how?
My suggestions are:
UPDATE can use index in WHERE clause (if the column in the clause has index)
INSERT can use index when uses SELECT (but in this case, index is from another table)
or probably when checking integrity constraints
but I don't have such deep knowledge of using indexes.
For UPDATE statements, index can be used by the optimiser if it deems the index can speed it up. The index would be used to locate the rows to be updated. The index is also a table in a manner of speaking, so if the indexed column is getting updated, it obviously needs to UPDATE the index as well. On the other hand if you're running an update without a WHERE clause the optimiser may choose not to use an index as it has to access the whole table, a full table scan may be more efficient (but may still have to update the index). The optimiser makes those decisions at runtime based on several parameters such as if there are valid stats against the tables and indexes in question, how much data is affected, what type of hardware, etc.
For INSERT statements though the INSERT itself does not need the index, the index will also need to be 'inserted into', so will need to be accessed by oracle. Another case where INSERT can cause the index to be used is an INSERT like this:
INSERT INTO mytable (mycolmn)
SELECT mycolumn + 10 FROM mytable;
Insert statement has no direct benefit for index. But more index on a table cause slower insert operation. Think about a table that has no index on it and if you want to add a row on it, it will find table block that has enough free space and store that row. But if that table has indexes on it database must make sure that these new rows also found via indexes, So to add new rows on a table that has indexes, also need to entry in indexes too. That multiplies the insert operation. So more index you have, more time you need to insert new rows.
For update it depends on whether you update indexed column or not. If you are not updating indexed column then performance should not be affected. Index can also speed up a update statements if the where conditions can make use of indexes.
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;
In our one of production database, we have 4 column table and there are no PK,UK constraints on it. only one notnull constraint on one column. The inserts are slow on this table and when I checked the indexes , there is one index which is built on all columns.
It is a normal table and not IOT. I really don't see a need of all column index, but wondering why the developers has created it?
Appreciate your thoughts?
It might be usefull, i.e. if you (mainly) query all columns oracle doesn't have to access the table at all, but can get all the data from the index. Though inserts take longer because a larger index has to be maintained by the dbms everytime.
One case where it could be useful is,
Say for example, you are trying to check the existence of records in this table and for that you have to have joins on all four columns. So in such a case if you have written a correlated query like below,
SELECT <something>
FROM table_1 t1
WHERE EXISTS
(SELECT 1 FROM table_t2 t2 where t1.c1=t2.c1 and t1.c2=t2.c2 and t1.c3=t2.c3 and t1.c4=t2.c4)
Apart from above case, it looks an error to me from developer's side.
Indexes are good to better query optimization but causes slow updates/inserts because the indexes needs to be updated at each modification.
If these tables first use is querying and inserts happens only in a specific periods like a batch at the beginning or the end of the day only, then you can remove the indexes before updating tables and then restore them.
In addition, all the queries all these tables need to be analysed to see which indexes are useful and which are not?
Anyway, You need to ask developers before removing these indexes.
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.