i was wondering, if i add one index for each field in every table of my DB, will that make my queries run faster?
or do i have to analyze my queries and create indexes only when required?
Adding an index on each column will probably make most of your queries faster, but it's not necessarily the best approach. It is better to tune your indexes to your specific queries, using EXPLAIN and performance measurements to guide you in adding the correct indexes.
In particular you need to understand when you shouldn't index a column, and when you need multi-column indexes.
I would advise reading the MySQL manual for optimization of SELECT statements which explains under what conditions indexes can be used.
The more indexes you have, the heavier inserting/updating gets. So it's a tradeoff. The select queries that cannot use an index now will get quicker ofcourse, but if you check what fields you're joining on (or using in a where) you will not trade off that much
(and, ofcourse, there is the disk-space, but most of the time I don't really care bout that: ) )
Another point is that MySql can only use a single index for a query, so if your query is
SELECT * FROM table WHERE status = 1 AND col1='foob' AND col2 = 'bar'
MySql will use 1 of the indexes, and filter out the rest reading the data from the table.
If you have queries like this, its better to create a composite index on (status, col1, col2)
Adding index on every field in every table is not smart.
You should add indexes ONLY on columns that you use in the WHERE clause in select OR on which you sort.
Often, the best results are achieved by using multi-column indexes that are specific to your SQL selects.
There are also a partial indexes with limit on the length of field which can also be used to optimize performance and reduce the index site.
Every unnecessary index will slow down the database during the insert because on every insert, every index has to be updated.
Also the more indexes you have, the more chances you have of data corruption. And lastly, indexes take extra storage space on disk, sometimes a lot of space.
Also MySQL tries to keep indexes in memory. If you have unnecessary indexes, there is a good change MySQL will end up using up the available memory with unnecessary indexes in which case your performance will degrade considerable.
Creating the right kind of indexes is probably the single most important optimization technique. That's why when someone asks something like this I thought it was a joke.
This question can only be asked by someone who have not read a single book on MySQL. Just get a good book and read it, then you will not have to ask questions like this.
Related
I'm experimenting with various indexing settings for my mysql database.
I wonder though, by removing or adding indexes is there any possibility to damage data rows in any way? Obviously I realise that if I make any application queries fail, that can cause bad rows. I'm more talking just about the structural queries themselves.
Or will I simply affect the efficiency of the database?
I just want to know if I have safety to experiment or if I have to be cautious?
The data isn't in phpmyadmin, it's in mysql. Adding/removing an index will not affect your data integrity by default. With a unique index, and using the ignore keyword it can.
That said - you should always have a backup of your data, it's easy to run a test like:
CREATE TABLE t1 LIKE t;
INSERT INTO t1 SELECT * FROM t;
ALTER TABLE t1 CREATE INDEX ...;
Then compare the difference in tables (perhaps a COUNT is fine in your case).
Adding/removing indexes is safe in terms of the rows in your table. However as you note, too many indexes or poorly constructed indexes can be (very) detrimental to performance. Likewise, adding indexes on large tables can be a very expensive process, and can bring a MySQL server to its knees, so you're better off not "experimenting" on production tables.
I need to set index to all the fields used for search a text given in search form.
In MySql, Is it possible to set indexes for all the fields of a table for speedup the select query. Is it good practice too?
Yes, it it possible to index all columns, but it's no good practice. More indexes maybe speed up your select and slow down insert, update, delete. First, the better way it it to check slow queries with mysql EXPLAIN to see what's going on.
I've seen this several times but I could be misinterpreting the EXPLAIN query plan.
Suppose I have a table(col1, col2).
I want to join it with another table on both col1 and col2.
So I create an index(col1, col2).
Sometimes, the EXPLAIN shows that the index is not being used. Perhaps some other inefficient index is used or none at all.
But if I create another index(col1), then the first index(col1, col2) is used.
Has anyone ever had this happen to them before? Do you have any idea why this might happen?
My theory is that the unused index provides some more accurate statistics about the table that hints to the query plan to use the first index. But I'm not familiar enough with the inner workings of mysql to know if this is true or how to prove it.
The documentation of MySQL for ALTER TABLE states that it may be required to run ANALYZE TABLE on it to refresh the index cardinality, which I believe to be a factor in the behaviour you're seeing. Also, the query optimiser usually handles empty (or near) empty tables quite different from populated tables, and it'll often do a full table scan instead of using an index when there are only a few rows. For my own development at $work I can't rely on the EXPLAIN output of my dev database because of that.
If i use
CREATE INDEX status_index ON eligible_users (status)
or
CREATE INDEX status ON eligible_users (status)
its the same thing no difference?
Also if i create alot of indexes will it actually help with queries or slow down?
Both statements you wrote do the same exact thing, only difference is the name of the index.
As for usefulness, it depends on the database setup and usage. Indexes are useful to speed up queries, but they have to be maintained on every INSERT/UPDATE, so it depends. There are a lot of resources available online about this wide topic.
An index can make or break a query. The execution time for certain queries can go from minutes to fractions of a second just by adding the correct indexes. In case you need to improve a query you can always prepend EXPLAIN to it, to see what MySQL's execution plan is: it will show what indexes the query uses (if any) and will help you troubleshoot some bottlenecks.
As said, an index is useful but is not free. It has to be kept up to date, so every time you insert or modify data in an indexed field, then the index must be updated too.
Generally in cases where you have a lot of reads and (relatively) few writes, indexes help a lot. But unnecessary indexes can degrade performance instead of improving it.
The short syntax for creating a single column index on column col from table tbl is:
CREATE INDEX [index_name] ON tbl (col)
Full details available in the MySQL Manual.
Is there any performance issues if you create an index with multiple columns, or should you do 1 index per column?
There's nothing inherently wrong with a multi-column index -- it depends completely on how you're going to query the data. If you have an index on colA+colB, it will help for queries like where colA='value' and colA='value' and colB='value' but it's not going to help for queries like colB='value'.
Advantages of MySQL Indexes
Generally speaking, MySQL indexing into database gives you three advantages:
Query optimization: Indexes make search queries much faster.
Uniqueness: Indexes like primary key index and unique index help to avoid duplicate row data.
Text searching: Full-text indexes in MySQL version 3.23.23, users have the opportunity to optimize searching against even large amounts of text located in any field indexed as such.
Disadvantages of MySQL indexes
When an index is created on the column(s), MySQL also creates a separate file that is sorted, and contains only the field(s) you're interested in sorting on.
Firstly, the indexes take up disk space. Usually the space usage isn’t significant, but because of creating index on every column in every possible combination, the index file would grow much more quickly than the data file. In the case when a table is of large table size, the index file could reach the operating system’s maximum file size.
Secondly, the indexes slow down the speed of writing queries, such as INSERT, UPDATE and DELETE. Because MySQL has to internally maintain the “pointers” to the inserted rows in the actual data file, so there is a performance price to pay in case of above said writing queries because every time a record is changed, the indexes must be updated. However, you may be able to write your queries in such a way that do not cause the very noticeable performance degradation.