I'm having trouble using a where clause on CREATE UNIQUE INDEX on mysql. I know you cannot just add where clause at the end of CREATE UNIQUE INDEX. Example below.
CREATE UNIQUE INDEX FAKE_TABLE_INDEX ON TABLE_NAME (COLUMN_NAME) WHERE INACTIVE = 0;
and this query above gives me an error. So is there alternative I can do to fix this query.
MySQL doesn't have filtered indexes. If I understand what they do (from reading the Microsoft docs) I think the closest analogous feature is multi-column indexes:
CREATE INDEX fake_table_index ON table_name (inactive, column_name);
This is more expensive than the filtered index because it indexes all the values of inactive, not just where inactive = 0.
This also doesn't have the unique constraint that the filtered index does. It's only useful for optimizing queries, not enforcing uniqueness. You'll have to do that with a trigger if you need it.
Related
Good morning.
I'm reading some examples of creating tables in mySQL but I'm not understanding the last statement on the following code: CREATE TABLE 'company_test', 'employee'( ... UNIQUE INDEX 'department_id_UNIQUE' ('department_id' ASC) VISIBLE) what does this UNIQUE INDEX 'department_id_UNIQUE ('department_id' ASC) VISIBLE do? I went searching everywhere that could explain this but I could only find exemples using CREATE INDEX outside CREATE TABLE, what does an index do and somebody can explain in detail that statement? If you could show me some documents talking about that I would be grateful.
1. What does this statement do?
UNIQUE INDEX 'department_id_UNIQUE ('department_id' ASC) VISIBLE
This statement creates unique index on department_id column of employee table in your example.
Index can be created during table creation as well as at later time after table is created.
MySQL Create table syntax: create table
Index creation after table creation: create index
2. What does index do?
Index improves read performance at the cost of write performance.In you example alongwith index it creates unique constraint on columns which will prevent duplicate values for department_id field.
This post explains how database indexing works in details.
UNIQUE INDEX 'department_id_UNIQUE ('department_id' ASC) VISIBLE
UNIQUE -- an index BTree is built and maintained; a uniqueness constraint is established.
INDEX -- optional (redundant) syntax
'...' -- optional arbitrary name for this index. A default will be provided if left out.
(...) -- The column(s) in the index. The combination is unique; the BTree is ordered by them.
ASC -- Ascending (as opposed to DESC: Descending)
VISIBLE -- below
VISIBLE is a new keyword. Some products tack this on; some other products see it as a syntax error. If that is a problem, simply remove the word.
The purpose of VISIBLE and INVISIBLE is thus. CREATEing and DROPping an index is potentially a costly action. If you want to remove the index (to save disk space), it can be advantageous to make it INVISIBLE to see if any query slows down drastically. That might tell you that the index is needed. Then you can quickly make it VISIBLE again. Else you can DROP it.
VISIBLE is the default; leaving it off is OK.
There is my SQL query. Table system_mailer is for logging sent e-mails. When i want to search some data, query is 10 seconds long. It is possible on any way to optimize this query?
SELECT `ID`
FROM `system_mailer`
WHERE `group` = 'selling_center'
AND `group_parameter_1` = '1'
AND `group_parameter_2` = '2138'
Timins is around couple of seconds, how could it be optimised?
You might find the following index on 4 columns would help performance:
CREATE INDEX idx ON system_mailer (`group`, group_parameter_1, group_parameter_2, ID);
MySQL should be able to use this index on your current query. By the way, if you are using InnoDB, and ID is the primary key, then you might be able to drop it from the explicit index definition, and just use this:
CREATE INDEX idx ON system_mailer (`group`, group_parameter_1, group_parameter_2);
Please avoid naming your columns and tables with reserved MySQL keywords like group. Because you made this design decision, you will now be forced to forever escape that column name with backticks (ugly).
just be sure you have a composite index on table system_mailer for the columns
(`group`, `group_parameter_1`, `group_parameter_2`)
and you can use redudancy adding the id to index for avoid data table access in query
(`group`, `group_parameter_1`, `group_parameter_2`, ID)
I am working with Couchbase 6.0. I know that we can create Secondary index on a filter.
So I have created a index like
CREATE INDEX idx_zipcode
ON userbucket(zipcode)
WHERE status = "active";
I have a question here:
Can I create an index on a filter clause if field is dynamic.
Something like this
CREATE INDEX idx_zipcode
ON userbucket(zipcode)
WHERE status = ? ;
Second question is,
which one is better in terms of performance:
Single index on 2 fields
CREATE INDEX idx_1 ON userbucket('fname','lname')
or
Separate Index on each field
CREATE INDEX idx_1 ON userbucket('fname')
CREATE INDEX idx_2 ON userbucket('lname')
No we can not create the index with the dynamic clause mentioned accepting a bind variable.
However, when it is sure that the status and zipcode are part of predicates and dynamic in nature the index like below would be handy.
CREATE INDEX idx_zipcode_status ON userbucket(zipcode, status);
Refer Couchbase Index Creation Blog - right performance.
Regarding the second query the same principle applies as
Index selection for a query solely depends on the filters in the WHERE
clause of your query
Secondary Composite index is also okay when you have both or leading columns in your query.
CREATE INDEX idx_1 ON userbucket('fname','lname')
The above index would be exploited by queries like:
SELECT * FROM userbucket WHERE fname= 'fnam' AND lname= 'lnam';
This question already has answers here:
When should I use a composite index?
(9 answers)
Closed 7 years ago.
In any relational Databases, we can create indexes that boost query speed. But creating more index can damage update/insert speed because the Db system will have to update each index when new data coming (insert, update, merge etc)
We use an example.
we can create a index called index1
ADD INDEX index1 (order_id ASC, buyer_id ASC)
OR we can create 2 indexes, index2 and index3
ADD INDEX index2 (order_id ASC)
ADD INDEX index3 (buyer_id ASC)
In a query like this
select * from tablename where order_id>100 and buyer_id>100
Which one is faster? By using Index1 or index2 and index3?
On the other side of the equation, when inserting or updating, I assume it will be much faster to just use one index instead of 2 but I haven't tested it against MySql or MSSQL server so I can't be so sure. If anyone has experience on that matter, do share it please.
And the last thing is about int typed values, I thought it's not possible or relevant to create a index just for int type columns because it doesn't boost the query time, is it true?
The performance of an index are linked to its selectivity, the fact of using two indexes, or a composite index must be assessed, in the context of its application or query particularly critical as regards the performance just by virtue of where on the fields as a possible the index reduces the number of rows to process (and put into joins).
In your case, since an order usually only one buyer is not very selective index order_id, buyer_id (pleasant its usefulness to join operations) as it would be more the reverse, buyer_id, order_id for facilitating the search for orders of a buyer
For the exact query you mentioned I would personally go for index1 (you will have a seek operation for both conditions at once). The same index should also do the job even if you filter by order_id only (because order id is the first column of the index, so the same BTREE structure should still help even if you omit the buyer).
At the same time index1 would not help much if you filter by buyer_id only (because the BTREE will be structured firstly by the missing order_id as per the index creation statement). You will probably end up with index scan with index1, while having separate indices would still work in that scenario (a seek on index3 is what should be expected).
I am having an issue with a table the uses a compound primary key.
The key consists of a date followed by an bigint.
Selects on the table look to be scanning even when only selecting fields from the PK and using a where clause that contains both columns. For Example
SELECT mydate, myid from foo WHERE mydate >='2014-08-26' AND my_id = 1234;
Explain select shows using where and the number of rows considered is in the millions.
One oddity is the key_len which is shown as 7 which seems far too small.
My instinct says the key is broken but I may be missing something obvious.
Any thoughts?
Thank you
Richard
For this query, the index you want is on id, date:
create index idx_foo_myid_mydate on foo(my_id, mydate);
This is because the conditions in the where clause have an equality and inequality. The equality conditions need to match the index from left to right, before the inequalities can be applied.
MySQL documentation actually does a good job (in my opinion) in explaining composite indexes.
Your existing index will be used for the inequality on mydate. However, all the index after the date in question will then be scanned to satisfy the condition on my_id. With the right index, MySQL can just go to the right rows directly.