How to select an sql index
I have an table TabEmp with fields c_ID (primary key), and c_Emp_ID.
I created an index on it idx_TabEmp (non-clustered) with the fields c_ID (primary key) and c_Emp_ID
I am using select statement
select * from index = idx_TabEmp
It throws an error
Incorrect syntax near 'index'. If this is intended as a part of a table hint, A WITH keyword and parenthesis are now required. See SQL Server Books Online for proper syntax
I am using
select * from TabEmp (index = idx_TabEmp)
it works, but I am not sure if that is the correct way to select an index
Can you please tell me the correct way to query an index?
The index is something which the optimizer picks up "automagically - ideally you don't need to force select an index.
If you really want to force select the index, use index hint
SELECT *
FROM TabEmp
WITH (INDEX(idx_TabEmp))
Also, note that with no filter conditions (ie, no WHERE clauses), the Index does not come into the picture since you are not searching for a specific data - you're selecting everything.
To provide the book analogy - when you're reading a full book cover to cover - you don't need to look at the Index. It's only when you're searching for a specific page that you look at the index and find what you want.
This is the syntax for a table hint.
SELECT column_list FROM table_name WITH (INDEX (index_name) [, ...]);
in sql server which I think is your case based on your error.
As to whether the index will be picked up or not (both in oracle and sql server) will depend on a lot of other reasons. As the name indicates, it is just a hint to the optimizer. The cost of the query using the hint and without the hint will eventually be the deciding factors for the optimizer.
In most cases, you'll not see the need to specify the hint. The optimizer used this access path if using the index is the best way to retrieve the data and all the meta data (statistics) indicate the same.
Related
I am using SQL Server over Azure and would like to give the ability to users to define keywords and, when an article where the title matches one of those keywords, the user would receive an alert.
Of course there can be 100.000 users each with 100 keywords defined.
To do such a query every time an article is inserted is obviously not feasible.
My idea is to create a job that would run every hour or so but, since for many reasons that also doesn't strike me as ideal, I was wondering if anyone would suggest a better option. Ideally using the azure infrastructure and not only a SQL based solution.
This is a question about how to query in SQL with large number of data.
According to your description, we can use database index to improve the performance of query.
We can create an index in the keyword field and use T-SQL as below:
Select count(1) From T Where Keyword = XXX
In this way, the database engine will use index instead of full table scan.
In Azure SQL Db, we can create index using T-SQL: CREATE INDEX (Transact-SQL)
We can also use SSMS to create index, more information about index in Azure SQL Db, we can refer to: Clustered and Nonclustered Indexes Described
Here are some optimization methods for database query for you, hope it will be help:
1. To optimize the query, avoid full table scanning as much as possible, and first consider indexing the columns involved in where and order by.
2. The null value judgment of the field in the where clause should be avoided as far as possible. Otherwise, it will cause the engine to abandon the index and scan the whole table, such as:
Select id from t where num is null
You can set a default value of 0 on num to ensure that there is no null value in the num column in the table.
Select id from t where num=0
3. try to avoid using "=" or "> operator in the where clause, otherwise the engine will discard the index and perform the full table scan.
4. Use or to join conditions in where clauses should be avoided as far as possible, otherwise it will cause the engine to abandon the use of the index and perform a full table scan, such as:
Select id from t where num=10 or num=20
It can be inquired like this:
Select id from t where num=10
Union all
Select id from t where num=20
5.in and not in should also be used with caution, otherwise the whole table will be scanned, such as:
Select id from t where num in (1,2,3)
For continuous values, use between instead of in:
Select id from t where num between 1 and 3
The query under
6. will also result in full table scan:
Select id from t where name like'%abc%'
In order to improve the efficiency, the full text retrieval can be considered.
You could use Logic Apps for your use-case.
There is a SQL connector in the logic app where you can achieve your requirements easily.
I've done a sample below
Explaination
Creating a trigger for your SQL Table when any item got inserted into a particular table (Customer_Feedback)
Execute a Stored Procedure (action) and get back the result/output from that.The Stored Procedure can be a simple select statement with your requirements for searching a keyword. Please be sure to follow the Indexing as per Lee Liu Answer above
Adding a condition which will check the output of the stored procedure with the corresponding Keyword
If the Condition Satisfies then send mail to that user via send mail task
You can also modify this flow with your own creativity.
Well i am new to this stuff ..I have created an index in my SP at start like follows
Create Index index_fab
ON TblFab (Fab_name)
Now i have query under this
select fab_name from TblFab where artc = 'x' and atelr = 'y'.
now Is it necessary to use this index name in select clause or it will automatically used to speed up queries
Do i have to use something like
select fab_name from TblFab WITH(INDEX(index_fab)) where artc = 'x' and atelr = 'y'.
or any other method to use this index in query
and also how to use index if we are using join on this table?
Firstly, do you mean you're creating the index in a stored procedure? That's a bad idea - if you run the stored procedure twice, it will fail because the index already exists.
Secondly, your query doesn't use the column mentioned in the index, so it will have no impact.
Thirdly, as JodyT writes, the query analyzer (SQL Server itself) will decide which index to use; it's almost certainly better at it than you are.
Finally, to speed up the query you mention, create an index on columns artc and atelr.
The Query Optimizer of SQL Server will decide if it the index is suitable for the query. You can't force it to use a specific index. You can give hints on which you want it to use but it won't be a guarantee that it will use it.
As the other people answered your question to help you to understand better, my opinion is, you should first understand why you need to use indexes. As we know that indexes increase the performance , they could also cause performance issues as well. Its better to know when you need to use indexes, why you need to use indexes instead of how to use indexes.
You can read almost every little detail from here .
Regarding your example, your query's index has no impact. Because it doesn't have the mentioned column in your query's where clause.
You can also try:
CREATE INDEX yourIndexName
ON yourTableName (column_you_are_looking_for1,column_you_are_lookingfor2)
Also good to know: If no index exists on a table, a table scan must be performed for each table referenced in a database query. The larger the table, the longer a table scan takes because a table scan requires each table row to be accessed sequentially. Although a table scan might be more efficient for a complex query that requires most of the rows in a table, for a query that returns only some table rows an index scan can access table rows more efficiently. (source from here )
Hope this helps.
An index should be used by default if you run a query against the table using it.
But I think in the query you posted it will not be used, because you are not filtering your data by the column you created your index on.
I think you would have to create the index for the artc and atelr columns to profit from that.
To see wether your index is used take a look at the execution plan that was used in the SQL Management Studio.
more info on indices: use the index luke
You dont need to include index in your query. Its managed by sql server. Also you dont need to include index in select if you want to make join to this table. Hope its clear.
You're index use "Fab_name" column which you don't filter on in your select statement, so it's of no use.
Since you're new to this, you might benefit from an index like this :
Create Index index_fab
ON TblFab (artc, atelr)
or maybe like this
Create Index index_fab
ON TblFab (atelr, artc)
...yes there are a lot of subtleties to learn.
For better performance:
List out the columns /tables which are frequently used,
Create index on those tables/columns only.
If index is properly set up, optimizer will use it automatically. By properly set up, I mean that it's selective enough, can effectively help the query etc. Read about it. You can check by yourself if index is being used by using "include actual execution plan" option in ssms.
It's generally not advised to use with(index()) hints and let optimizer decided by itself, except from very special cases when you just know better ;).
I built sqlite3 from source to include the FTS3 support and then created a new table in an existing sqlite database containing 1.5million rows of data, using
CREATE VIRTUAL TABLE data USING FTS3(codes text);
Then used
INSERT INTO data(codes) SELECT originalcodes FROM original_data;
Then queried each table with
SELECT * FROM original_data WHERE originalcodes='RH12';
This comes back instantly as I have an index on that column
The query on the FTS3 table
SELECT * FROM data WHERE codes='RH12';
Takes almost 28 seconds
Can someone help explain what I have done wrong as I expected this to be significantly quicker
The documentation explains:
FTS tables can be queried efficiently using SELECT statements of two different forms:
Query by rowid. If the WHERE clause of the SELECT statement contains a sub-clause of the form "rowid = ?", where ? is an SQL expression, FTS is able to retrieve the requested row directly using the equivalent of an SQLite INTEGER PRIMARY KEY index.
Full-text query. If the WHERE clause of the SELECT statement contains a sub-clause of the form " MATCH ?", FTS is able to use the built-in full-text index to restrict the search to those documents that match the full-text query string specified as the right-hand operand of the MATCH clause.
If neither of these two query strategies can be used, all queries on FTS tables are implemented using a linear scan of the entire table.
For an efficient query, you should use
SELECT * FROM data WHERE codes MATCH 'RH12'
but this will find all records that contain the search string.
To do 'normal' queries efficiently, you have to keep a copy of the data in a normal table.
(If you want to save space, you can use a contentless or external content table.)
You should read documentation more carefully.
Any query against virtual FTS table using WHERE col = 'value' will be slow (except for query against ROWID), but query using WHERE col MATCH 'value' will be using FTS and fast.
I'm not an expert on this, but here are a few things to think about.
Your test is flawed (I think). You are contrasting a scenario where you have an exact text match (the index can be used on original_data - nothing is going to outperform this scenario) with an equality on the fts3 table (I'm not sure that FTS3 would even come into play in this type of query). If you want to compare apples to apples (to see the benefit of FTS3), you're going to want to compare a "like" operation on original_data against the FTS3 "match" operation on data.
Lets say in the employee table, I have created an index(idx_name) on the emp_name column of the table.
Do I need to explicitly specify the index name in select clause or it will automatically used to speed up queries.
If it is required to be specified in the select clause, What is the syntax for using index in select query ?
If you want to test the index to see if it works, here is the syntax:
SELECT *
FROM Table WITH(INDEX(Index_Name))
The WITH statement will force the index to be used.
Good question,
Usually the DB engine should automatically select the index to use based on query execution plans it builds. However, there are some pretty rare cases when you want to force the DB to use a specific index.
To be able to answer your specific question you have to specify the DB you are using.
For MySQL, you want to read the Index Hint Syntax documentation on how to do this
How to use index in select statement? this way:
SELECT * FROM table1 USE INDEX (col1_index,col2_index)
WHERE col1=1 AND col2=2 AND col3=3;
SELECT * FROM table1 IGNORE INDEX (col3_index)
WHERE col1=1 AND col2=2 AND col3=3;
SELECT * FROM t1 USE INDEX (i1) IGNORE INDEX (i2) USE INDEX (i2);
And many more ways check this
Do I need to explicitly specify?
No, no Need to specify explicitly.
DB engine should automatically select the index to use based on query execution plans it builds from #Tudor Constantin answer.
The optimiser will judge if the use of your index will make your query run faster, and if it is, it will use the index. from #niktrl answer
In general, the index will be used if the assumed cost of using the index, and then possibly having to perform further bookmark lookups is lower than the cost of just scanning the entire table.
If your query is of the form:
SELECT Name from Table where Name = 'Boris'
And 1 row out of 1000 has the name Boris, it will almost certainly be used. If everyone's name is Boris, it will probably resort to a table scan, since the index is unlikely to be a more efficient strategy to access the data.
If it's a wide table (lot's of columns) and you do:
SELECT * from Table where Name = 'Boris'
Then it may still choose to perform the table scan, if it's a reasonable assumption that it's going to take more time retrieving the other columns from the table than it will to just look up the name, or again, if it's likely to be retrieving a lot of rows anyway.
The optimiser will judge if the use of your index will make your query run faster, and if it is, it will use the index.
Depending on your RDBMS you can force the use of an index, although it is not recommended unless you know what you are doing.
In general you should index columns that you use in table join's and where statements
Generally, when you create an index on a table, database will automatically use that index while searching for data in that table. You don't need to do anything about that.
However, in MSSQL, you can specify an index hint which can specify that a particular index should be used to execute this query. More information about this can be found here.
Index hint is also seems to be available for MySQL. Thanks to Tudor Constantine.
By using the column that the index is applied to within your conditions, it will be included automatically. You do not have to use it, but it will speed up queries when it is used.
SELECT * FROM TABLE WHERE attribute = 'value'
Will use the appropriate index.
The index hint is only available for Microsoft Dynamics database servers.
For traditional SQL Server, the filters you define in your 'Where' clause should persuade the engine to use any relevant indices...
Provided the engine's execution plan can efficiently identify how to read the information (whether a full table scan or an indexed scan) - it must compare the two before executing the statement proper, as part of its built-in performance optimiser.
However, you can force the optimiser to scan by using something like
Select *
From [yourtable] With (Index(0))
Where ...
Or to seek a particular index by using something like
Select *
From [yourtable] With (Index(1))
Where ...
The choice is yours. Look at the table's index properties in the object panel to get an idea of which index you want to use. It ought to match your filter(s).
For best results, list the filters which would return the fewest results first.
I don't know if I'm right in saying, but it seems like the query filters are sequential; if you get your sequence right, the optimiser shouldn't have to do it for you by comparing all the combinations, or at least not begin the comparison with the more expensive queries.
I have a column in a non-partitioned Oracle table defined as VARCHAR2(50); the column has a standard b-tree index. I was wondering if there is an optimal way to query this column to determine whether it contains a given value. Here is the current query:
SELECT * FROM my_table m WHERE m.my_column LIKE '%'||v_value||'%';
I looked at Oracle Text, but that seems like overkill for such a small column. However, there are millions of records in this table so looking for substring matches is taking more time than I'd like. Is there a better way?
No.
That query is a table scan. If v_value is an actual word, then you may very well want to look at Oracle Text or a simple inverted index scheme you roll your on your own. But as is, it's horrible.
Oracle Text covers a number of different approaches, not all of them heavyweight. As your column is quite small you could index it with a CTXCAT index.
SELECT * FROM my_table m
WHERE catsearch(m.my_column, v_value, null) > 0
/
Unlike the other type of Text index, CTXCAT indexes are transactional, so they do not require synchronisation. Such indexes consume a lot of space, but that you have to pay some price for improved performance.
Find out more.
You have three choices:
live with it;
use something like Oracle Text for full-text searching; or
redefine the problem so you can implement a faster solution.
The simplest way to redefine the problem is to say the column has to start with the search term (so lose the first %), which will then use the index.
An alternative way is to say that the search starts on word boundaries (so "est" will match "estimate" but not "test"). MySQL (MyISAM) and SQL Server have functions that will do matching like this. Not sure if Oracle does. If it doesn't you could create a lookup table of words to search instead of the column itself and you could populate that table on a trigger.
You could put a function-based index on the column, using the REGEXP_LIKE function. You might need to create the fbi with a case statement to return '1' with a match, as boolean returning functions dont seem to be valid in fbi.
Here is an example.
Create the index:
CREATE INDEX regexp_like_on_myCol ON my_table (
CASE WHEN REGEXP_LIKE(my_column, '[static exp]', 'i')
THEN 1
END);
And then to use it, instead of:
SELECT * FROM my_table m WHERE m.my_column LIKE '%'||v_value||'%';
you will need to perform a query like the following:
SELECT * FROM my_table m WHERE (
CASE WHEN REGEXP_LIKE(m.my_column, '[static exp]', 'i')
THEN 1
END) IS NOT NULL;
A significant shortcomming in this approach is that you will need to know your '[static exp]' at the time that you create your index. If you are looking for a performance increase while performing ad hoc queries, this might not be the solution for you.
A bonus though, as the function name indicates, is that you have the opportunity to create this index using regex, which could be a powerful tool in the end. The evaluation hit will be taken when items are added to the table, not during the search.
You could try INSTR:
...WHERE INSTR(m.my_column, v_value) > 0
I don't have access to Oracle to test & find out if it is faster than LIKE with wildcarding.
For the most generic case where you do not know in advance the string you are searching for then the best access path you can hope for is a fast full index scan. You'd have to focus on keeping the index as small as possible, which might have it's own problems of course, and could look at a compressed index if the data is not very high cardinality.