I got confused with Visible INDEX and Invisible INDEX in Mysql. Which is best?
I have a database tables with rows about 1M rows. The previous development team doesn't add Index to the table columns. Now we need to add Index to some columns to speed up the fetch query.
While Indexing the column I got confused with Visible and Invisible columns.
Kindly anyone explain me which is best choice. I tried to learn from online tutorials but I can't get it clear.
A question comes up while developing a database: "Do we really need all these indexes?"
Indexes cost some overhead to keep in sync with data. If the index is needed to optimize some queries, it's generally worth keeping the index. But if the index is not used by any query, it might be time to drop that index. But how can you be sure it's not needed?
Why not test by dropping the index and recreating it if it turns out the index is needed? Because tables get really, really large, and adding the index back could take a long time. In one case, one of the developers I supported dropped an index he thought was not needed. It turned out it was important. But adding the index back took four weeks because the table was so huge, and the server was running hard just keeping up with the other queries. In the meantime, the query that needed that index was running extremely poorly.
It would have been preferable to somehow make MySQL temporarily pretend the index does not exist, and then flip a switch and make the index visible again when they decided it was important.
https://dev.mysql.com/doc/refman/8.0/en/invisible-indexes.html says:
MySQL supports invisible indexes; that is, indexes that are not used by the optimizer.
Invisible indexes make it possible to test the effect of removing an index on query performance, without making a destructive change that must be undone should the index turn out to be required. Dropping and re-adding an index can be expensive for a large table, whereas making it invisible and visible are fast, in-place operations.
An invisible index is an index that exists, and is still kept in sync with data as you run INSERT/UPDATE/DELETE statements. But the optimizer treats it as if the index is not there.
An index hint in a query can suppress use of any index, and that's the way we had to test during MySQL 5.x. But that requires finding all your SQL queries that would use the index, and changing application code. That's especially inconvenient if your queries are generated by layers of ORM code.
In simple words
Visible INDEX are working or Active Index.
Invisible INDEX are removed Indexes, which were marked as Removed.
Related
Does a database have to rebuild its indexes every time a new row is inserted?
And by that token, wouldn't it mean if I was inserting alot, the index would be being rebuilt constantly and therefore less effective/useless for querying?
I'm trying to understand some of this database theory for better database design.
Updates definitely don't require rebuilding the entire index every time you update it (likewise insert and delete).
There's a little bit of overhead to updating entries in an index, but it's reasonably low cost. Most indexes are stored internally as a B+Tree data structure. This data structure was chosen because it allows easy modification.
MySQL also has a further optimization called the Change Buffer. This buffer helps reduce the performance cost of updating indexes by caching changes. That is, you do an INSERT/UPDATE/DELETE that affects an index, and the type of change is recorded in the Change Buffer. The next time you read that index with a query, MySQL reads the Change Buffer as a kind of supplement to the full index.
A good analogy for this might be a published document that periodically publishes "errata" so you need to read both the document and the errata together to understand the current state of the document.
Eventually, the entries in the Change Buffer are gradually merged into the index. This is analogous to the errata being edited into the document for the next time the document is reprinted.
The Change Buffer is used only for secondary indexes. It doesn't do anything for primary key or unique key indexes. Updates to unique indexes can't be deferred, but they still use the B+Tree so they're not so costly.
If you do OPTIMIZE TABLE or some types of ALTER TABLE changes that can't be done in-place, MySQL does rebuild the indexes from scratch. This can be useful to defragment an index after you delete a lot of the table, for example.
Yes, inserting affects them, but it's not as bad as you seem to think. Like most entities in relational databases, indexes are usually created and maintained with an extra amount of space to accommodate for growth, and usually set up to increase that extra amount automatically when index space is nearly exhausted.
Rebuilding the index starts from scratch, and is different from adding entries to the index. Inserting a new row does not result in the rebuild of an index. The new entry gets added in the extra space mentioned above, except for clustered indexes which operate a little differently.
Most DB administrators also do a task called "updating statistics," which updates an internal set of statistics used by the query planner to come up with good query strategies. That task, performed as part of maintenance, also helps keep the query optimizer "in tune" with the current state of indexes.
There are enormous numbers of high-quality references on how databases work, both independent sites and those of the publishers of major databases. You literally can make a career out of becoming a database expert. But don't worry too much about your inserts causing troubles. ;) If in doubt, speak to your DBA if you have one.
Does that help address your concerns?
Let's say I have a table with 1M rows and I need to add an index on it. What would be the process of doing so? Am I able to do ALTER TABLE table ADD INDEX column (column) directly on production? Or do I need to take other things into account to add the index.
How does one normally go about adding an index on a live production db?
You should first try adding the same index on your test database under similar load conditions and check that it doesn't cause problems. It's possible that by creating the index you lock the table for some time and cause other queries to fail.
One million rows is a large table, but it's not huge. You will probably find that the adding the index completes reasonably quickly. Unless you have real-time constraints it's unlikely to cause serious issues. It's definitely worth testing it first though.
Personally I'd wait until things were quiet, but you don't have to take the database offline. Users might see a delay, but it shouldn't time out. (If it does, something else is wrong.)
Note that adding an index doesn't make the dbms copy the whole table (as adding a column might).
I have a table which already contains an index in MySQL. I added some rows to the table, do I need to re-index the table somehow or does MySQL do this for me automatically?
This would be done automatically. This is the reason, why sometimes we don't want to create indexes -- rebuilding of parts of indexes on inserting have small but not empty overhead in performance.
If you define an index in MySQL then it will always reflect the current state of the database unless you have deliberately disabled indexing. As soon as indexing is re-enabled, the index will be brought up to date. Usually indexing is only disabled during large insertions for performance reasons.
There is a cost associated with each index on your table. While a good index can speed up retrieval times immensely, every index you define slows insertion by a small amount. The insertion costs grow slowly with the size of the database. This is why you should only define indexes you absolutely need if you are going to be working on large sets of data.
If you want to see what indexes are defined, you can use SHOW CREATE TABLE to have a look at a particular table.
No, you didn't need to rebuild index.
Record insertion will automatically affect old index..
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.
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.