Here is my situation. I have a MySQL MyISAM table containing about 4 million records with a total of 13,3 GB of data. The table contains messages received from an external system. Two of the columns in the table keep track of a timestamp and a boolean whether the message is handled or not.
When using this query:
SELECT MIN(timestampCB) FROM webshop_cb_onx_message
The result shows up almost instantly.
However, I need to find the earliest timestamp of unhandled messages, like this:
SELECT MIN(timestampCB ) FROM webshop_cb_onx_message WHERE handled = 0
The results of this query show up after about 3 minutes, which is way too slow for the script I'm writing.
Both columns are individually indexed, not together. However, adding an index to the table would take incredibly long considering the amount of data that is in there already.
Does my problem originate from the fact that both columns are separatly indexed, and if so, does anyone have a solution to my issue other than adding another index?
It is commonly recommended that if the selectivity of an index over 20% then a full table scan is preferable over an index access. This would mean it is likely that your index on handled won't actually result in using the index but a full table scan given the selectivity.
A composite index of handled, timestampCB may actually improve the performance given its a composite index, even if the selectivity isn't great MySQL would most likely still use it - even if it didn't you could force it's use.
Related
I've been using indexes on my MySQL databases for a while now but never properly learnt about them. Generally I put an index on any fields that I will be searching or selecting using a WHERE clause but sometimes it doesn't seem so black and white.
What are the best practices for MySQL indexes?
Example situations/dilemmas:
If a table has six columns and all of them are searchable, should I index all of them or none of them?
What are the negative performance impacts of indexing?
If I have a VARCHAR 2500 column which is searchable from parts of my site, should I index it?
You should definitely spend some time reading up on indexing, there's a lot written about it, and it's important to understand what's going on.
Broadly speaking, an index imposes an ordering on the rows of a table.
For simplicity's sake, imagine a table is just a big CSV file. Whenever a row is inserted, it's inserted at the end. So the "natural" ordering of the table is just the order in which rows were inserted.
Imagine you've got that CSV file loaded up in a very rudimentary spreadsheet application. All this spreadsheet does is display the data, and numbers the rows in sequential order.
Now imagine that you need to find all the rows that have some value "M" in the third column. Given what you have available, you have only one option. You scan the table checking the value of the third column for each row. If you've got a lot of rows, this method (a "table scan") can take a long time!
Now imagine that in addition to this table, you've got an index. This particular index is the index of values in the third column. The index lists all of the values from the third column, in some meaningful order (say, alphabetically) and for each of them, provides a list of row numbers where that value appears.
Now you have a good strategy for finding all the rows where the value of the third column is "M". For instance, you can perform a binary search! Whereas the table scan requires you to look N rows (where N is the number of rows), the binary search only requires that you look at log-n index entries, in the very worst case. Wow, that's sure a lot easier!
Of course, if you have this index, and you're adding rows to the table (at the end, since that's how our conceptual table works), you need to update the index each and every time. So you do a little more work while you're writing new rows, but you save a ton of time when you're searching for something.
So, in general, indexing creates a tradeoff between read efficiency and write efficiency. With no indexes, inserts can be very fast -- the database engine just adds a row to the table. As you add indexes, the engine must update each index while performing the insert.
On the other hand, reads become a lot faster.
Hopefully that covers your first two questions (as others have answered -- you need to find the right balance).
Your third scenario is a little more complicated. If you're using LIKE, indexing engines will typically help with your read speed up to the first "%". In other words, if you're SELECTing WHERE column LIKE 'foo%bar%', the database will use the index to find all the rows where column starts with "foo", and then need to scan that intermediate rowset to find the subset that contains "bar". SELECT ... WHERE column LIKE '%bar%' can't use the index. I hope you can see why.
Finally, you need to start thinking about indexes on more than one column. The concept is the same, and behaves similarly to the LIKE stuff -- essentially, if you have an index on (a,b,c), the engine will continue using the index from left to right as best it can. So a search on column a might use the (a,b,c) index, as would one on (a,b). However, the engine would need to do a full table scan if you were searching WHERE b=5 AND c=1)
Hopefully this helps shed a little light, but I must reiterate that you're best off spending a few hours digging around for good articles that explain these things in depth. It's also a good idea to read your particular database server's documentation. The way indices are implemented and used by query planners can vary pretty widely.
Check out presentations like More Mastering the Art of Indexing.
Update 12/2012: I have posted a new presentation of mine: How to Design Indexes, Really. I presented this in October 2012 at ZendCon in Santa Clara, and in December 2012 at Percona Live London.
Designing the best indexes is a process that has to match the queries you run in your app.
It's hard to recommend any general-purpose rules about which columns are best to index, or whether you should index all columns, no columns, which indexes should span multiple columns, etc. It depends on the queries you need to run.
Yes, there is some overhead so you shouldn't create indexes needlessly. But you should create the indexes that give benefit to the queries you need to run quickly. The overhead of an index is usually far outweighed by its benefit.
For a column that is VARCHAR(2500), you probably want to use a FULLTEXT index or a prefix index:
CREATE INDEX i ON SomeTable(longVarchar(100));
Note that a conventional index can't help if you're searching for words that may be in the middle of that long varchar. For that, use a fulltext index.
I won't repeat some of the good advice in other answers, but will add:
Compound Indices
You can create compound indices - an index that includes multiple columns. MySQL can use these from left to right. So if you have:
Table A
Id
Name
Category
Age
Description
if you have a compound index that includes Name/Category/Age in that order, these WHERE clauses would use the index:
WHERE Name='Eric' and Category='A'
WHERE Name='Eric' and Category='A' and Age > 18
but
WHERE Category='A' and Age > 18
would not use that index because everything has to be used from left to right.
Explain
Use Explain / Explain Extended to understand what indices are available to MySQL and which one it actually selects. MySQL will only use ONE key per query.
EXPLAIN EXTENDED SELECT * from Table WHERE Something='ABC'
Slow Query Log
Turn on the slow query log to see which queries are running slow.
Wide Columns
If you have a wide column where MOST of the distinction happens in the first several characters, you can use only the first N characters in your index. Example: We have a ReferenceNumber column defined as varchar(255) but 97% of the cases, the reference number is 10 characters or less. I changed the index to only look at the first 10 characters and improved performance quite a bit.
If a table has six columns and all of them are searchable, should i index all of them or none of them
Are you searching on a field by field basis or are some searches using multiple fields?
Which fields are most being searched on?
What are the field types? (Index works better on INTs than on VARCHARs for example)
Have you tried using EXPLAIN on the queries that are being run?
What are the negetive performance impacts of indexing
UPDATEs and INSERTs will be slower. There's also the extra storage space requirments, but that's usual unimportant these days.
If i have a VARCHAR 2500 column which is searchable from parts of my site, should i index it
No, unless it's UNIQUE (which means it's already indexed) or you only search for exact matches on that field (not using LIKE or mySQL's fulltext search).
Generally I put an index on any fields that i will be searching or selecting using a WHERE clause
I'd normally index the fields that are the most queried, and then INTs/BOOLEANs/ENUMs rather that fields that are VARCHARS. Don't forget, often you need to create an index on combined fields, rather than an index on an individual field. Use EXPLAIN, and check the slow log.
Load Data Efficiently: Indexes speed up retrievals but slow down inserts and deletes, as well as updates of values in indexed columns. That is, indexes slow down most operations that involve writing. This occurs because writing a row requires writing not only the data row, it requires changes to any indexes as well. The more indexes a table has, the more changes need to be made, and the greater the average performance degradation. Most tables receive many reads and few writes, but for a table with a high percentage of writes, the cost of index updating might be significant.
Avoid Indexes: If you don’t need a particular index to help queries perform better, don’t create it.
Disk Space: An index takes up disk space, and multiple indexes take up correspondingly more space. This might cause you to reach a table size limit more quickly than if there are no indexes. Avoid indexes wherever possible.
Takeaway: Don't over index
In general, indices help speedup database search, having the disadvantage of using extra disk space and slowing INSERT / UPDATE / DELETE queries. Use EXPLAIN and read the results to find out when MySQL uses your indices.
If a table has six columns and all of them are searchable, should i index all of them or none of them?
Indexing all six columns isn't always the best practice.
(a) Are you going to use any of those columns when searching for specific information?
(b) What is the selectivity of those columns (how many distinct values are there stored, in comparison to the total amount of records on the table)?
MySQL uses a cost-based optimizer, which tries to find the "cheapest" path when performing a query. And fields with low selectivity aren't good candidates.
What are the negetive performance impacts of indexing?
Already answered: extra disk space, lower performance during insert - update - delete.
If i have a VARCHAR 2500 column which is searchable from parts of my site, should i index it?
Try the FULLTEXT Index.
1/2) Indexes speed up certain select operations but they slow down other operations like insert, update and deletes. It can be a fine balance.
3) use a full text index or perhaps sphinx
I have a Postgres table with several columns, one column is the datetime that the column was last updated. My query is to get all the updated rows between a start and end time. It is my understanding for this query to use WHERE in this query instead of BETWEEN. The basic query is as follows:
SELECT * FROM contact_tbl contact
WHERE contact."UpdateTime" >= '20150610' and contact."UpdateTime" < '20150618'
I am new at creating SQL queries, I believe this query is doing a full table scan. I would like to optimize it if possible. I have placed a Normal index on the UpdateTime column, which takes a long time to create, but with this index the query is faster. One thing I am not sure about is if have to keep recalculating this index if the table gets bigger/columns get changed. Also, I am considering a CLUSTERED index on the UpdateTime row, but I wanted to ask if there was a canonical way of optimizing this/if I was on the right track first
Placing an index on UpdateTime is correct. It will allow the index to be used instead of full table scans.
2 WHERE conditions like the above vs. using the BETWEEN keyword are the exact same:
http://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#operator_between
BETWEEN is just "syntactical sugar" for those that like that syntax better.
Indexes allow for faster reads, but slow down writes (because like you mention, the new data has to be inserted into the index as well). The entire index does not need to be recalculated. Indexes are smart data structures, so the extra data can be added without a lot of extra work, but it does take some.
You're probably doing many more reads than writes, so using an index is a good idea.
If you're doing lots of writes and few reads, then you'd want to think a bit more about it. It would then come down to business requirements. Although overall the throughput may be slowed, read latency may not be a requirement but write latency may be, in which case you wouldn't want the index.
For instance, think of this lottery example: Everytime someone buys a ticket, you have to record their name and the ticket number. However the only time you ever have to read that data, is after the 1 and only drawing to see who had that ticket number. In this database, you wouldn't want to index the ticket number since they'll be so many writes and very few reads.
I have a field table_name in a table which can have only 20 different values. The total records in the table is about few tens of thousands of rows. If I do a query like this:
SELECT * FROM table WHERE table_name = 'adasd';
at most the returned records are 25% of the total rows. Mostly I get only 10% of the total records. Is there a scope to index the field table_name here? I hear that for indexes to work well it requires the values in that field to be unique or close to it. In my case, its not at all close to unique. But I also heard that if the returned rows are less in number compared to total number of rows, it makes a good case for indexing.
How should I go about this?
No they don't have to be unique to get a benefit from using indexes, however take some time to think about what the DBMS does when processing a query:
Full table scan - a sequential read through the data (i.e. very few seek operations)
Index lookup - a few seeks on the index to find the start of the selected data, then a sequential read (few seeks) to identify rows in the underlying table, then LOTS AND LOTS of seeks to fetch the rows from the table
Seeks are expensive.
(there is a secondary effect of full table scans in that they are more prone to flushing hot data out of the cache - but you should address the primary concern first).
In this case, it's unlikely that the DBMS would use the index if it were present, and even if it did, it would probably be slower than a full table scan. As a (very) rough rule of thumb, you're only going to get a benefit from an index if a predicate identifies less than around 5% of the rows (but it will vary depending on the relative size of the index and the data).
i.e. don't bother adding an index on this field alone.
I think you may benefit from spending some time thinking about why you need to run queries which return so many rows?
Revised Answer
I just learned that creating an index does not mean that MySQL will use it. Keeping that in mind, I will re-phrase my answer:
You should create an index on that column if (general or your own) practices suggest you to do so. MySQL will use heuristics; which include looking at the available indexes and their respective cardinality, to determine the best index to use or not to use an index at all.
Interesting reading about this topic here.
I need to add indexes to my table (columns) and stumbled across this post:
How many database indexes is too many?
Quote:
“Having said that, you can clearly add a lot of pointless indexes to a table that won't do anything. Adding B-Tree indexes to a column with 2 distinct values will be pointless since it doesn't add anything in terms of looking the data up. The more unique the values in a column, the more it will benefit from an index.”
Is an Index really pointless if there are only two distinct values? Given a table as follows (MySQL Database, InnoDB)
Id (BIGINT)
fullname (VARCHAR)
address (VARCHAR)
status (VARCHAR)
Further conditions:
The Database contains 300 Million records
Status can only be “enabled” and “disabled”
150 Million records have status= enabled and 150 Million records have
stauts= disabled
My understanding is, without having an index on status, a select with where status=’enabled’ would result in a full tablescan with 300 Million Records to process?
How efficient is the lookup when I use a BTREE index on status?
Should I index this column or not?
What alternatives (maybe any other indexes) does MySQL InnoDB provide to efficiently look records up by the "where status="enabled" clause in the given example with a very low cardinality/selectivity of the values?
The index that you describe is pretty much pointless. An index is best used when you need to select a small number of rows in comparison to the total rows.
The reason for this is related to how a database accesses a table. Tables can be assessed either by a full table scan, where each block is read and processed in turn. Or by a rowid or key lookup, where the database has a key/rowid and reads the exact row it requires.
In the case where you use a where clause based on the primary key or another unique index, eg. where id = 1, the database can use the index to get an exact reference to where the row's data is stored. This is clearly more efficient than doing a full table scan and processing every block.
Now back to your example, you have a where clause of where status = 'enabled', the index will return 150m rows and the database will have to read each row in turn using separate small reads. Whereas accessing the table with a full table scan allows the database to make use of more efficient larger reads.
There is a point at which it is better to just do a full table scan rather than use the index. With mysql you can use FORCE INDEX (idx_name) as part of your query to allow comparisons between each table access method.
Reference:
http://dev.mysql.com/doc/refman/5.5/en/how-to-avoid-table-scan.html
I'm sorry to say that I do not agree with Mike. Adding an index is meant to limit the amount of full records searches for MySQL, thereby limiting IO which usually is the bottleneck.
This indexing is not free; you pay for it on inserts/updates when the index has to be updated and in the search itself, as it now needs to load the index file (full text index for 300M records is probably not in memory). So it might well be that you get extra IO in stead of limitting it.
I do agree with the statement that a binary variable is best stored as one, a bool or tinyint, as that decreases the length of a row and can thereby limit disk IO, also comparisons on numbers are faster.
If you need speed and you seldom use the disabled records, you may wish to have 2 tables, one for enabled and one for disabled records and move the records when the status changes. As it increases complexity and risk this would be my very last choice of course. Definitely do the move in 1 transaction if you happen to go for it.
It just popped into my head that you can check wether an index is actually used by using the explain statement. That should show you how MySQL is optimizing the query. I don't really know hoe MySQL optimizes queries, but from postgresql I do know that you should explain a query on a database approximately the same (in size and data) as the real database. So if you have a copy on the database, create an index on the table and see wether it's actually used. As I said, I doubt it, but I most definitely don't know everything:)
If the data is distributed like 50:50 then query like where status="enabled" will avoid half scanning of the table.
Having index on such tables is completely depends on distribution of data, i,e : if entries having status enabled is 90% and other is 10%. and for query where status="disabled" it scans only 10% of the table.
so having index on such columns depends on distribution of data.
#a'r answer is correct, however it needs to be pointed out that the usefulness of an index is given not only by its cardinality but also by the distribution of data and the queries run on the database.
In OP's case, with 150M records having status='enabled' and 150M having status='disabled', the index is unnecessary and a waste of resource.
In case of 299M records having status='enabled' and 1M having status='disabled', the index is useful (and will be used) in queries of type SELECT ... where status='disabled'.
Queries of type SELECT ... where status='enabled' will still run with a full table scan.
You will hardly need all 150 mln records at once, so I guess "status" will always be used in conjunction with other columns. Perhaps it'd make more sense to use a compound index like (status, fullname)
Jan, you should definitely index that column. I'm not sure of the context of the quote, but everything you said above is correct. Without an index on that column, you are most certainly doing a table scan on 300M rows, which is about the worst you can do for that data.
Jan, as asked, where your query involves simply "where status=enabled" without some other limiting factor, an index on that column apparently won't help (glad to SO community showed me what's up). If however, there is a limiting factor, such as "limit 10" an index may help. Also, remember that indexes are also used in group by and order by optimizations. If you are doing "select count(*),status from table group by status", an index would be helpful.
You should also consider converting status to a tinyint where 0 would represent disabled and 1 would be enabled. You're wasting tons of space storing that string vs. a tinyint which only requires 1 byte per row!
I have a similar column in my MySQL database. Approximately 4 million rows, with the distribution of 90% 1 and 10% 0.
I've just discovered today that my queries (where column = 1) actually run significantly faster WITHOUT the index.
Foolishly I deleted the index. I say foolishly, because I now suspect the queries (where column = 0) may have still benefited from it. So, instead I should explicitly tell MySQL to ignore the index when I'm searching for 1, and to use it when I'm searching for 0. Maybe.
I'm trying to fine-tune my MySQL server so I check my settings, analyzing slow-query log, and simplify my queries if possible.
Sometimes it is enough if I am indexing correctly, sometimes not. I've read somewhere (please correct me if this is stupidity) that more indexes than I need make the same effect, like if I don't have any of indexes.
How many indexes are enough? You can say it depends on hundreds of factors, but I'm curious about how can I clean up my mysql-slow.log enough to reduce server load.
Furthermore, I saw some "interesting" log entries like this:
# Query_time: 0 Lock_time: 0 Rows_sent: 22 Rows_examined: 44
SELECT * FROM `categories` ORDER BY `orderid` ASC;
The table in question contains exactly 22 rows, index set in orderid. Why is this query showing up in the log after all? Why examine 44 rows if it only contains 22?
The amount of indexing and the line of doing too much will depend on a lot of factors. On small tables like your "categories" table you usually don't want or need an index and it can actually hurt performance. The reason being is that it takes I/O (i.e. time) to read an index and then more I/O and time to retrieve the records associated with the matched rows. An exception being when you only query the columns contained within the index.
In your example you are retrieving all the columns and with only 22 rows and it may be faster to just do a table scan and sort those instead of using the index. The optimizer may/should be doing this and ignoring the index. If that is the case, then the index is just taking up space with no benefit. If your "categories" table is accessed often, you may want to consider pinning it into memory so the db server keeps it accessible without having to goto the disk all the time.
When adding indexes you need to balance out disk space, query performance, and the performance of updating and inserting into the tables. You can get away with more indexes on tables that are static and don't change much as opposed to tables with millions of updates a day. You'll start feeling the affects of index maintenance at that point. What is acceptable in your environment though is and can only be determined by you and your organization.
When doing your analysis, be sure to generate/update your table and index statistics so that you can be assured of accurate calculations.
As a general rule, you should have indexes on all primary keys (you don't have a choice in that), all foreign keys, and any other fields you commonly use to fetch rows.
For example, if I commonly look up users by username, I would have that indexed, even if user ID was the primary key.
How many indexes depends entirely on the queries your running, what kinds of joins are being done (if any), the kind of data stored in the table and how big the tables are (as well as many other factors). There's really no exact science to it. The greatest tool in your arsenal for figuring out how to optimize a query is explain. Using explain you can find out what kind of joins are being down, what possible keys could be used and which key (if any) was used as well as how many rows were examined for each table in the join.
Using this information you can decide how to key your tables and/or modify your queries to make them more efficient. The syntax for explain is very simple.
EXPLAIN SELECT * FROM `categories` ORDER BY `orderid` ASC;
Note, explain does not actually run the query. So if you're using this to debug a query that takes 5 minutes to run, explain will still be very fast.
You do need to be careful when adding indexes though as they do cause inserts and updates to go slower and on very large tables this performance hit can become noticeable. Especially if that same table is used for a lot of reads. While adding a lot of indexes generally won't kill the performance of a query, you should still only add them as yo
Also keep in mind that MySQL will use a maximum of one index per select statement (although if you are using a join, it can also use one for each join). So indexing just because is a waste of disk space and will slow the database down on writes. If you commonly use a where statement on two columns, do one index containing both of those columns, it will be significantly faster than indexing just one alone.
An index can speed up a SELECT query, but it will slow down INSERT/UPDATE/DELETE queries because they need to update the index as well, not just the row.
This is just personal opinion (I've got no facts to back it up), but I think that if there is a query that is taking a long time and an index would speed it up - go for it! "Too many" indexes would be if you added indexes that didn't do any good (e.g. there were no queries it would speed up). For example, a silly thing to do would be to place an index on every column "just because".
There's no magic number for the "best" number of indexes. The basic rule is this: add indexes for queries that are used often and/or need to run quickly.
Having "too many" indexes shouldn't slow down queries, but it each index added adds a small amount of time to add/update items in the db (since it modifies the indices as well), and a small amount of space. However, if you're just adding indexes as required, this is probably not a big concern.