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..
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?
Do I need to re-index the MySQL table if I insert new records into it? Table has existing indexes.
No, because indexes are updated automatically on any change.
MySQL won't need to re-index the entire table when you insert a new record. It will however need to create an index entry for your newly inserted record.
There is therefore a slight performance impact in creating indexes on tables, in that inserts will take slightly longer. In most practical situations this is acceptable given the big performance improvement you get on reading data from indexed tables.
There is more information available at https://dev.mysql.com/doc/refman/5.0/en/insert-speed.html
I have a table in my MySQL database with round 5M rows. Inserting rows to the table is too slow as MySQL updates index while inserting. How to stop index updating while inserting and do the indexing separately later?
Thanks
Kamrul
Sounds like your table might be over indexed. Maybe post your table definition here so we can have a look.
You have two choices:
Keep current indexes and remove unused indexes. If you have 3 indexes on a table for every single write to the table there will be 3 writes to the indexes. A index is only helpful during reads so you might want to remove unused indexes. During a load indexes will be updated which will slow down your load.
Drop you indexes before load then recreate them after load. You can drop your indexes before data load then insert and rebuild. The rebuild might take longer than the slow inserts. You will have to rebuild all indexes one by one. Also unique indexes can fail if duplicates are loaded during the load process without the indexes.
Now I suggest you take a good look at the indexes on the table and reduce them if they are not used in any queries. Then try both approaches and see what works for you. There is no way I know of in MySQL to disable indexes as they need the values insert to be written to their internal structures.
Another thing you might want to try it to split the IO over multiple drives i.e partition your table over several drives to get some hardware performance in place.
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 have a table with about 200,000 records. i want to add a field to it:
ALTER TABLE `table` ADD `param_21` BOOL NOT NULL COMMENT 'about the field' AFTER `param_20`
but it seems a very heavy query and it takes a very long time, even on my Quad amd PC with 4GB of RAM.
i am running under windows/xampp and phpMyAdmin.
does mysql have a business with every record when adding a field?
or can i change the query so it makes the change more quickly?
MySQL will, in almost all cases, rebuild the table during an ALTER**. This is because the row-based engines (i.e. all of them) HAVE to do this to retain the data in the right format for querying. It's also because there are many other changes you could make which would also require rebuilding the table (such as changing indexes, primary keys etc)
I don't know what engine you're using, but I will assume MyISAM. MyISAM copies the data file, making any necessary format changes - this is relatively quick and is not likely to take much longer than the IO hardware can get the old datafile in and the new on out to disc.
Rebuilding the indexes is really the killer. Depending on how you have it configured, MySQL will either: for each index, put the indexed columns into a filesort buffer (which may be in memory but is typically on disc), sort that using its filesort() function (which does a quicksort by recursively copying the data between two files, if it's too big for memory) and then build the entire index based on the sorted data.
If it can't do the filesort trick, it will just behave as if you did an INSERT on every row, and populate the index blocks with each row's data in turn. This is painfully slow and results in far from optimal indexes.
You can tell which it's doing by using SHOW PROCESSLIST during the process. "Repairing by filesort" is good, "Repairing with keycache" is bad.
All of this will use AT MOST one core, but will sometimes be IO bound as well (especially copying the data file).
** There are some exceptions, such as dropping secondary indexes on innodb plugin tables.
You add a NOT NULL column, the tuples need to be populated. So it will be slow...
This touches each of 200.000 records, as each record needs to be updated with a new bool value which is not going to be null.
So; yes it's an expensive query... There is nothing you can do to make it faster.