I'm trying to query using mysql FULLTEXT, but unfortunately its returning empty results even the table contain those input keyword.
Table: user_skills:
+----+----------------------------------------------+
| id | skills |
+----+----------------------------------------------+
| 1 | Dance Performer,DJ,Entertainer,Event Planner |
| 2 | Animation,Camera Operator,Film Direction |
| 3 | DJ |
| 4 | Draftsman |
| 5 | Makeup Artist |
| 6 | DJ,Music Producer |
+----+----------------------------------------------+
Indexes:
Query:
SELECT id,skills
FROM user_skills
WHERE ( MATCH (skills) AGAINST ('+dj' IN BOOLEAN MODE))
Here once I run the above query none of the DJ rows are returning. In the table there are 3 rows with is having the value dj.
A full text index is the wrong approach for what you are trying to do. But, your specific issue is the minimum word length, which is either 3 or 4 (by default), depending on the ending. This is explained in the documentation, specifically here.
Once you reset the value, you will need to recreate the index.
I suspect you are trying to be clever. You have probably heard the advice "don't store lists of things in delimited strings". But you instead countered "ah, but I can use a full text index". You can, although you will find that more complex queries do not optimize very well.
Just do it right. Create the association table user_skills with one row per user and per skill that the user has. You will find it easier to use in queries, to prevent duplicates, to optimize queries, and so on.
Your search term is to short
as in mysql doc
Some words are ignored in full-text searches:
Any word that is too short is ignored. The default minimum length of words that are found by full-text searches is three characters for
InnoDB search indexes, or four characters for MyISAM. You can control
the cutoff by setting a configuration option before creating the
index: innodb_ft_min_token_size configuration option for InnoDB search
indexes, or ft_min_word_len for MyISAM.
.
Boolean full-text searches have these characteristics:
They do not use the 50% threshold.
They do not automatically sort rows in order of decreasing relevance.
You can see this from the preceding query result: The row with the
highest relevance is the one that contains “MySQL” twice, but it is
listed last, not first.
They can work even without a FULLTEXT index, although a search
executed in this fashion would be quite slow.
The minimum and maximum word length full-text parameters apply.
https://dev.mysql.com/doc/refman/5.6/en/fulltext-natural-language.html
https://dev.mysql.com/doc/refman/5.6/en/fulltext-boolean.html
Related
I'm currently studying about MySQL command and got stuck at using the "MATCH...AGAINST" command on FULLTEXT index. It returns an "empty set" when it's against a "stopword"(which is "and" in my case).
Here's what I did. The database I'm working on contains a list of books and their author. I'm trying to select the entries that contain "and" in their title. Here's a list in my 'classics' table.
+--------------------+------------------------------+
| author | title |
+--------------------+------------------------------+
| Mark Twain | The Adventures of Tom Sawyer |
| Jane Austen | Pride and Prejudice |
| Charles Darwin | The Origin of Species |
| Charles Dickens | The Old Curiosity Shop |
| William Shakespear | Romeo and Juliet |
+--------------------+------------------------------+
This is the code I've written
SELECT author, title FROM classics
WHERE MATCH(author, title) AGAINST('and');
Empty set (0.00 sec)
The result in my expectation was "Pride and Prejudice" and "Romeo and Juliet" instead of "Empty set (0.00 sec)". I now realized that "and" is a stopword.
My question is What does the "stopword" mean and how do I know which word is a stopword? And what should I do if I really want to select the query which contains "and" in its title?
My question is What does the "stopword" mean ...
A stopword is a word that will be ignored when given as a keyword in a full-text search.
For more information read the Wikipedia page on stopwords.
MySQL uses the term in a way that is consistent with the normal definition.
... and how do I know which word is a stopword?
For InnoDB tables you can query the INFORMATION_SCHEMA.INNODB_FT_DEFAULT_STOPWORD table.
For MyISAM search indexes, the stopwords are loaded from a file. It may be possible to read the file at runtime using Java file I/O, but it apparently can't be accessed via a database query.
And what should I do if I really want to select the query which contains "and" in its title?
The MySQL documentation explains how to do it; see Section 12.9.4 Full-Text Stopwords. (There is too much detail to copy it here.)
My reading is that you need to make configuration changes and restart the database server to change the stopwords. For InnoDB tables you also need to regenerate the table's full-text index.
That means that you cannot change the stopwords for each query ... if that is what you are aiming to do. But you could explicitly query for a stopword using LIKE; e.g.
SELECT author, title FROM classics
WHERE title LIKE '% and %';
That query would probably entail a table scan, so you want to avoid it if possible.
You can see an example of stopword list in dev.mysql.com:
To see the default InnoDB stopword list, query the INFORMATION_SCHEMA.INNODB_FT_DEFAULT_STOPWORD table.
mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_FT_DEFAULT_STOPWORD;
+-------+
| value |
+-------+
| a |
| about |
See more at "The INFORMATION_SCHEMA INNODB_FT_DEFAULT_STOPWORD Table"
The glossary defines stopword as:
In a FULLTEXT index, a word that is considered common or trivial enough that it is omitted from the search index and ignored in search queries.
Different configuration settings control stopword processing for InnoDB and MyISAM tables.
To force a fulltext index to include three letters words, you would need to change ft_min_word_len to 3 (restart mysqld and rebuild the table)
Maybe you should just do like:
SELECT author, title FROM classics WHERE title LIKE '% and %';
I have a table with huge number of records. When I query from that specific table specially when using ORDER BY in query, it takes too much execution time.
How can I optimize this table for Sorting & Searching?
Here is an example scheme of my table (jobs):
+---+-----------------+---------------------+--+
| id| title | created_at |
+---+-----------------+---------------------+--+
| 1 | Web Developer | 2018-04-12 10:38:00 | |
| 2 | QA Engineer | 2018-04-15 11:10:00 | |
| 3 | Network Admin | 2018-04-17 11:15:00 | |
| 4 | System Analyst | 2018-04-19 11:19:00 | |
| 5 | UI/UX Developer | 2018-04-20 12:54:00 | |
+---+-----------------+---------------------+--+
I have been searching for a while, I learned that creating INDEX can help improving the performance, can someone please elaborate how the performance can be increased?
Add "explain" word before ur query, and check result
explain select ....
There u can see what u need to improve, then add index on ur search and/or sorting field and run explain query again
If you want to earn performance on your request, a way is paginating it. So, you can put a limit (as you want) and specify the page you want to display.
For example SELECT * FROM your_table LIMIT 50 OFFSET 0.
I don't know if this answer will help you in your problem but you can try it ;)
Indexes are the databases way to create lookup trees (B-Trees in most cases) to more efficiently sort, filter, and find rows.
Indexes are used to find rows with specific column values quickly.
Without an index, MySQL must begin with the first row and then read
through the entire table to find the relevant rows. The larger the
table, the more this costs. If the table has an index for the columns
in question, MySQL can quickly determine the position to seek to in
the middle of the data file without having to look at all the data.
This is much faster than reading every row sequentially.
https://dev.mysql.com/doc/refman/5.5/en/mysql-indexes.html
You can use EXPLAIN to help identify how the query is currently running, and identify areas of improvement. It's important to not over-index a table, for reasons probably beyond the scope of this question, so it'd be good to do some research on efficient uses of indexes.
ALTER TABLE jobs
ADD INDEX(created_at);
(Yes, there is a CREATE INDEX syntax that does the equivalent.)
Then, in the query, do
ORDER BY created_at DESC
However, with 15M rows, it may still take a long time. Will you be filtering (WHERE)? LIMITing?
If you really want to return to the user 15M rows -- well, that is a lot of network traffic, and that will take a long time.
MySQL details
Regardless of the index declaration or version, the ASC/DESC in ORDER BY will be honored. However it may require a "filesort" instead of taking advantage of the ordering built into the index's BTree.
In some cases, the WHERE or GROUP BY is to messy for the Optimizer to make use of any index. But if it can, then...
(Before MySQL 8.0) While it is possible to declare an index DESC, the attribute is ignored. However, ORDER BY .. DESC is honored; it scans the data backwards. This also works for ORDER BY a DESC, b DESC, but not if you have a mixture of ASC and DESC in the ORDER BY.
MySQL 8.0 does create DESC indexes; that is, the BTree containing the index is stored 'backwards'.
Suppose you have a FULLTEXT index defined on a column in MySQL database table to allow for natural language searches. If you now run a query using MATCH() and AGAINST(), you can retrieve the "rank" of the search results, as described here:
https://dev.mysql.com/doc/refman/5.6/en/fulltext-natural-language.html
For example:
mysql> SELECT id, body, MATCH (title,body) AGAINST
('Security implications of running MySQL as root'
IN NATURAL LANGUAGE MODE) AS score
FROM articles WHERE MATCH (title,body) AGAINST
('Security implications of running MySQL as root'
IN NATURAL LANGUAGE MODE);
+----+-------------------------------------+-----------------+
| id | body | score |
+----+-------------------------------------+-----------------+
| 4 | 1. Never run mysqld as root. 2. ... | 1.5219271183014 |
| 6 | When configured properly, MySQL ... | 1.3114095926285 |
+----+-------------------------------------+-----------------+
2 rows in set (0.00 sec)
The problem is that MATCH() returns some floating point number but no upper bound to it. I need to derive a "confidence factor" to each of the resulting rows as a percentage 0 to 100. For example, a confidence factor of 95% for a particular row would mean that it's very likely exactly what the user is searching for. Conversely, if the confidence factor is low, it'd be something like 10%.
Note that this is not a matter of selecting the larges score from MATCH() and setting that to 100. The row with the largest score may still be not at all what the user is searching for... So perhaps using MATCH() won't work but, could you please suggest some way to calculate such a "confidence factor"?
Much thanks in advance.
Two rows in the my database have the following data:
brand | product | style
=================================================
Doc Martens | Doc Martens 1460 Boots | NULL
NewBalance | New Balance WR1062 SG Width | NULL
Mininum word length is set to 3, and a FULLTEXT index is created across all the three columns above.
When I run a search for IS BOOLEAN matches for +doc in the index, I get the first row returned as a result. When I search for +new, I get no results.
Can someone explain why?
Thanks.
It called FULLTEXT because of it indexes whole words. so for searching words started with "New" you have to put "astrisk" sign in the end:
MATCH (`brand`) AGAINST ('new*')
More detailed here
A colleague asked me to explain how indexes (indices?) boost up performance; I tried to do so, but got confused myself.
I used the model below for explanation (an error/diagnostics logging database). It consists of three tables:
List of business systems, table "System" containing their names
List of different types of traces, table "TraceTypes", defining what kinds of error messages can be logged
Actual trace messages, having foreign keys from System and TraceTypes tables
I used MySQL for the demo, however I don't recall the table types I used. I think it was InnoDB.
System TraceTypes
----------------------------- ------------------------------------------
| ID | Name | | ID | Code | Description |
----------------------------- ------------------------------------------
| 1 | billing | | 1 | Info | Informational mesage |
| 2 | hr | | 2 | Warning| Warning only |
----------------------------- | 3 | Error | Failure |
| ------------------------------------------
| ------------|
Traces | |
--------------------------------------------------
| ID | System_ID | TraceTypes_ID | Message |
--------------------------------------------------
| 1 | 1 | 1 | Job starting |
| 2 | 1 | 3 | System.nullr..|
--------------------------------------------------
First, i added some records to all of the tables and demonstrated that the query below executes in 0.005 seconds:
select count(*) from Traces
inner join System on Traces.System_ID = System.ID
inner join TraceTypes on Traces.TraceTypes_ID = TraceTypes.ID
where
System.Name='billing' and TraceTypes.Code = 'Info'
Then I generated more data (no indexes yet)
"System" contained about 100 entries
"TraceTypes" contained about 50 entries
"Traces" contained ~10 million records.
Now the previous query took 8-10 seconds.
I created indexes on Traces.System_ID column and Traces.TraceTypes_ID column. Now this query executed in milliseconds:
select count(*) from Traces where System_id=1 and TraceTypes_ID=1;
This was also fast:
select count(*) from Traces
inner join System on Traces.System_ID = System.ID
where System.Name='billing' and TraceTypes_ID=1;
but the previous query which joined all the three tables still took 8-10 seconds to complete.
Only when I created a compound index (both System_ID and TraceTypes_ID columns included in index), the speed went down to milliseconds.
The basic statement I was taught earlier is "all the columns you use for join-ing, must be indexed".
However, in my scenario I had indexes on both System_ID and TraceTypes_ID, however MySQL didn't use them. The question is - why? My bets is - the item count ratio 100:10,000,000:50 makes the single-column indexes too large to be used. But is it true?
First, the correct, and the easiest, way to analyze a slow SQL statement is to do EXPLAIN. Find out how the optimizer chose its plan and ponder on why and how to improve that. I'd suggest to study the EXPLAIN results with only 2 separate indexes to see how mysql execute your statement.
I'm not very familiar with MySQL, but it seems that there's restriction of MySQL 4 of using only one index per table involved in a query. There seems to be improvements on this since MySQL 5 (index merge), but I'm not sure whether it applies to your case. Again, EXPLAIN should tell you the truth.
Even with using 2 indexes per table allowed (MySQL 5), using 2 separate indexes is generally slower than compound index. Using 2 separate indexes requires index merge step, compared to the single pass of using a compound index.
Multi Column indexes vs Index Merge might be helpful, which uses MySQL 5.4.2.
It's not the size of the indexes so much as the selectivity that determines whether the optimizer will use them.
My guess would be that it would be using the index and then it might be using traditional look up to move to another index and then filter out. Please check the execution plan. So in short you might be looping through two indexes in nested loop. As per my understanding. We should try to make a composite index on column which are in filtering or in join and then we should use Include clause for the columns which are in select. I have never worked in MySql so my this understanding is based on SQL Server 2005.