I want to search in a MySQL table in two columns for one or more keywords.
I tried this query:
select *
from table
where match(col1, col2) against ('keyword1 keyword 2' with query expansion)
But this query does not always gives the right results. Any suggestions?
May be this helps:
select *, match(col1, col2)
against ('keyword1 keyword2' in boolean mode) as relevance
from content
where match(col1, col2)
against ('keyword1 keyword2' in boolean mode)
order by relevance;
Compare results of this query and the one you wrote above.
Note that using WITH QUERY EXPANSION results in a more fuzzy search. From the documentation:
"It works by performing the search twice, where the search phrase for the second search is the original search phrase concatenated with the few most highly relevant documents from the first search."
Related
I have table with quite many rows (~2M).
When i search it like
SELECT * FROM product WHERE
MATCH(name) AGAINST('+some' '+word' IN BOOLEAN MODE)
it works like charm and finds what i need in less than 0.5s.
But when i search for 2 sets of words like this
SELECT * FROM product WHERE
MATCH(name) AGAINST('+some' '-word' IN BOOLEAN MODE)
OR
MATCH(name) AGAINST('+something' '-other' IN BOOLEAN MODE)
search takes sometimes over minute.
I would expect it to work 2 times slower (it's 2 searches), maybe a bit more (you still have to compare results and remove duplicates, but if there are only few results it should not take long), but not so much longer. After adding OR it works slower, than LIKE "%...%" OR LIKE "%...%"
Anyone can tell me what am i doing wrong?
Unfortunately for you, fulltext indexes have some limitations, and not being able to properly merge the results of two independent fulltext searches is one them:
The Index Merge optimization algorithm has the following known limitations:
[...]
Index Merge is not applicable to full-text indexes.
Fortunately for you, fulltext searches can be more complex, so you can merge your searches yourself. Your second query can be written as a single search using:
SELECT * FROM product WHERE
MATCH(name) AGAINST('(+something -other) (+some -word)' IN BOOLEAN MODE)
This defines two search sets and is ok if either of the two (...) matches - which is an or.
Alternatively, you can use a union instead of an or, which allows MySQL to actually run two independent fulltext searches and then combine the two results, which is basically what you are thinking of:
SELECT * FROM product WHERE
MATCH(name) AGAINST('+some -word' IN BOOLEAN MODE)
UNION
SELECT * FROM product WHERE
MATCH(name) AGAINST('+something -other' IN BOOLEAN MODE)
This also works for more complicated situations, e.g. merging searches on different columns, but will not work that easy if you want to do something else than or.
I have a MySQL MyISAM table with a full text index on the keywords column and 20 millions rows. It works well when a search for rare words, for example:
SELECT count(*) FROM books WHERE MATCH(keywords) AGAINST ('+DUCK' IN BOOLEAN MODE)
(0.005s, 2k results)
But when I search for a more common terms it is much slowers:
SELECT count(*) FROM books WHERE MATCH(keywords) AGAINST ('+YES' IN BOOLEAN MODE)
(5s, 2millions results)
It makes sens because the last one returns much more rows, but then how can I pre-filter the rows before the text search? This doesn't work:
SELECT count(*) FROM books WHERE date > "2019-09-23" AND MATCH(keywords) AGAINST ('+YES' IN BOOLEAN MODE)
(5s, 0 result)
MyISAM's (and maybe InnoDB's) FULLTEXT will always do the MATCH first, then any other clauses. Hence, adding that extra filter does not help with speed.
Think of it this way... A FT index is constructed to test the entire table(s) for the MATCH clause. It is not ready to handle any filtering before it goes to work. So, you are stuck with FT first, then filter the results the other way but without benefit of any indexes.
I want to find all rows that match a full-text search for one pair of columns but also do not match the same text in another column.
Both of these seem to work
SELECT * FROM docs WHERE MATCH(title, descript) AGAINST ('energy' IN BOOLEAN MODE) AND NOT MATCH(categories) AGAINST ('energy' IN BOOLEAN MODE);
Or using a subquery:
SELECT * FROM docs WHERE MATCH(title, descript) AGAINST ('energy' IN BOOLEAN MODE) AND id NOT IN (SELECT id FROM docs where MATCH(categories) AGAINST ('energy' IN BOOLEAN MODE));
The docs field has the relevant full-text indexes set up.
Any reason to prefer one over the other?
On the (small) database I'm using they are both very fast, too fast to measure reliably.
Thanks for any suggestions.
How can I do a MySQL search which will match partial words but also provide accurate relevancy sorting?
SELECT name, MATCH(name) AGAINST ('math*' IN BOOLEAN MODE) AS relevance
FROM subjects
WHERE MATCH(name) AGAINST ('math*' IN BOOLEAN MODE)
The problem with boolean mode is the relevancy always returns 1, so the sorting of results isn't very good. For example, if I put a limit of 5 on the search results the ones returned don't seem to be the most relevant sometimes.
If I search in natural language mode, my understanding is that the relevancy score is useful but I can't match partial words.
Is there a way to perform a query which fulfils all of these criteria:
Can match partial words
Results are returned with accurate relevancy
Is efficient
The best I've got so far is:
SELECT name
FROM subjects
WHERE name LIKE 'mat%'
UNION ALL
SELECT name
FROM subjects
WHERE name LIKE '%mat%' AND name NOT LIKE 'mat%'
But I would prefer not to be using LIKE.
The new InnoDB full-text search feature in MySQL 5.6 helps in this case.
I use the following query:
SELECT MATCH(column) AGAINST('(word1* word2*) ("word1 word1")' IN BOOLEAN MODE) score, id, column
FROM table
having score>0
ORDER BY score
DESC limit 10;
where ( ) groups words into a subexpression. The first group has like word% meaning; the second looks for exact phrase. The score is returned as float.
I obtained a good solution in this (somewhat) duplicate question a year later:
MySQL - How to get search results with accurate relevance
Well i'm running 2 queries that should show me the same result,
First query:
SELECT count( id ) AS cv FROM table_name WHERE field_name LIKE '%êêê01, word02, word03%'
Second query:
SELECT count( id ) AS cv FROM table_name WHERE match(field_name) against('êêê01, word02, word03')
but the first show more rows than the second, someone could tell me why?
I'm using fulltext index on this field,
Thanks.
I did a quick research and the following quote should answer your question:
One problem with MATCH on MySQL is that it seems to only match against whole words so a search for 'bla' won't match a column with a value of 'blah'.
It's also described in the documentation for match
By default, the MATCH() function performs a natural language search for a string against a text collection. A collection is a set of one or more columns included in a FULLTEXT index. The search string is given as the argument to AGAINST(). For each row in the table, MATCH() returns a relevance value; that is, a similarity measure between the search string and the text in that row in the columns named in the MATCH() list.
Meanwhile like is more "powerful" as it can look upon individuals characters:
Per the SQL standard, LIKE performs matching on a per-character basis, thus it can produce results different from the = comparison operator:
Which explains why like returns more results than match.