I have a query which looks like:
SELECT *
FROM my_table
WHERE 'some_string' LIKE mytable.some_column || '%%'
How can I index some_column to improve this query performance?
Or is the a better way to filter this?
This predicate effectively searches for all prefixes for a given string:
WHERE 'some_string' LIKE mytable.some_column || '%'
Maybe % is a special character in your client, which needs to be escaped with another % to pass a literal %, else '%%' would be just noise and can be replaced with '%'.
The most efficient solution should be a recursive CTE (or similar) that matches to every prefix exactly, starting with some_column = left('some_string', 1), up to some_column = left('some_string', length('some_string')) (= 'some_string').
You only need a plain btree index on the column for this. Depending on details of your implementation, partial expression indexes might improve performance ...
Related:
Reverse pattern matching: find the longest prefix
Algorithm for finding the longest prefix
PostgreSQL LIKE query performance variations
Pattern matching with LIKE, SIMILAR TO or regular expressions in PostgreSQL
I believe you intend to write the following query:
SELECT *
FROM my_table
WHERE mytable.some_column LIKE 'some_string%';
In other words, you want to find records where some column begins with some_string followed by anything, possibly nothing at all.
As far as I know, a regular B-tree index on some_column will be effective, to a point, in your query. The reason is that Postgres can traverse the tree looking for the prefix some_string. Once it has found that entry, beyond that the index might not help. But an index on some_column should give you some performance benefit here.
A condition where an index would not help would be the following:
WHERE mutable.some_column LIKE '%some_string';
In this case, the index is rendered mostly useless, because we have no idea with what prefix the column value should begin.
Related
I have a table which contains a text column, say vehicle number.
Now I want to query the table for fields which contain a particular vehicle number.
While matching I do not want to consider non-alphanumeric characters.
example: query condition - DEL123
should match - DEL-123, DEL/123, DEL#123, etc...
If you know which characters to skip, put them as the second parameter of this translate() call (which is faster than regexp functions):
select *
from a_table
where translate(code, '-/#', '') = 'DEL123';
Else, you can compare only alphanumeric characters using regexp_replace():
select *
from a_table
where regexp_replace(code, '[^[:alnum:]]', '', 'g') = 'DEL123';
#klin's answer is great, but is not sargable, so in cases where you're searching through millions of records (maybe not your case, but perhaps someone else with a similar question looking for answers), using regular expressions will likely render much better results.
The following will use indexes on code significantly reducing the number of rows tested:
select *
from a_table
where code ~ '^DEL[^[:alnum:]]*123$';
As we known, there're prefix length column which can be added during index creation. For example in mysql,
alter table j1 add index idx_j1_str1 (str1(5));
I didn't find any equivalent solution in postgresql after I searched the google.com and stackoverflow.com.
Then can anyone tell me the answer except the function index in postgresql.
Any reply will be appreciated.
create index idx_j1_str on j1 (left(str1,5));
But I don't think you need something like this in Postgres. An index on just str1 is probably much more versatile. But of course this depends heavily on the queries you run - which you did not show us, so it's impossible to say what kind of index you really need.
To make use of a function based index in Postgres (and basically any other DBMS that supports them) your query needs to contain the same expression as you used in the index:
select *
from j1
where left(str1,5) = '1234'
will use the above index (if it makes sense, e.g. if the table is large enough and the condition reduces the overall result substantially).
If you create a regular index on that column:
create index idx_j1_str on j1 (str1 varchar_pattern_ops);
then it can be used for something like:
select *
from j1
where str1 like '1234%'
(which is equivalent to left(str1,5) = '1234') but it can also be used for:
select *
from j1
where str1 like '1234678%'
or any other prefix search using a wildcard.
I know about full-text search, but that only matches your query against individual words. I want to select strings that contain a word that starts with words in my query. For example, if I search:
appl
the following should match:
a really nice application
apples are cool
appliances
since all those strings contains words that start with appl. In addition, it would be nice if I could select the number of words that match, and sort based on that.
How can I implement this in PostgreSQL?
Prefix matching with Full Text Search
FTS supports prefix matching. Your query works like this:
SELECT * FROM tbl
WHERE to_tsvector('simple', string) ## to_tsquery('simple', 'appl:*');
Note the appended :* in the tsquery. This can use an index.
See:
Get partial match from GIN indexed TSVECTOR column
Alternative with regular expressions
SELECT * FROM tbl
WHERE string ~ '\mappl';
Quoting the manual here:
\m .. matches only at the beginning of a word
To order by the count of matches, you could use regexp_matches()
SELECT tbl_id, count(*) AS matches
FROM (
SELECT tbl_id, regexp_matches(string, '\mappl', 'g')
FROM tbl
WHERE string ~ '\mappl'
) sub
GROUP BY tbl_id
ORDER BY matches DESC;
Or regexp_split_to_table():
SELECT tbl_id, string, count(*) - 1 AS matches
FROM (
SELECT tbl_id, string, regexp_split_to_table(string, '\mappl')
FROM tbl
WHERE string ~ '\mappl'
) sub
GROUP BY 1, 2
ORDER BY 3 DESC, 2, 1;
db<>fiddle here
Old sqlfiddle
Postgres 9.3 or later has index support for simple regular expressions with a trigram GIN or GiST index. The release notes for Postgres 9.3:
Add support for indexing of regular-expression searches in pg_trgm
(Alexander Korotkov)
See:
PostgreSQL LIKE query performance variations
Depesz wrote a blog about index support for regular expressions.
SELECT * FROM some_table WHERE some_field LIKE 'appl%' OR some_field LIKE '% appl%';
As for counting the number of words that match, I believe that would be too expensive to do dynamically in postgres (though maybe someone else knows better). One way you could do it is by writing a function that counts occurrences in a string, and then add ORDER BY myFunction('appl', some_field). Again though, this method is VERY expensive (i.e. slow) and not recommended.
For things like that, you should probably use a separate/complimentary full-text search engine like Sphinx Search (google it), which is specialized for that sort of thing.
An alternative to that, is to have another table that contains keywords and the number of occurrences of those keywords in each string. This means you need to store each phrase you have (e.g. really really nice application) and also store the keywords in another table (i.e. really, 2, nice, 1, application, 1) and link that keyword table to your full-phrase table. This means that you would have to break up strings into keywords as they are entered into your database and store them in two places. This is a typical space vs speed trade-off.
I have a query with a number of test fields something like this:
SELECT * FROM some-table
WHERE field1 ILIKE "%thing%"
OR field2 ILIKE "%thing"
OR field3 ILIKE "%thing";
The columns are pretty much all varchar(50) or thereabouts. Now I understand to improve performance I should index the fields upon which the search operates. Should I be considering replacing ILIKE with TSEARCH completely?
A full text search setup is not identical to a "contains" like query. It stems words etc so you can match "cars" against "car".
If you really want a fast ILIKE then no standard database index or FTS will help. Fortunately, the pg_trgm module can do that.
http://www.postgresql.org/docs/9.1/static/pgtrgm.html
http://www.depesz.com/2011/02/19/waiting-for-9-1-faster-likeilike/
One thing that is very important: NO B-TREE INDEX will ever improve this kind of search:
where field ilike '%SOMETHING%'
What I am saying is that if you do a:
create index idx_name on some_table(field);
The only access you will improve is where field like 'something%'. (when you search for values starting with some literal). So, you will get no benefit by adding a regular index to field column in this case.
If you need to improve your search response time, definitely consider using FULL TEXT SEARCH.
Adding a bit to what the others have said.
First you can't really use an index based on a value in the middle of the string. Indexes are tree searches generally, and you have no way to know if your search will be faster than just scanning the table, so PostgreSQL will default to a seq scan. Indexes will only be used if they match the first part of the string. So:
SELECT * FROM invoice
WHERE invoice_number like 'INV-2012-435%'
may use an index but like '%44354456%' cannot.
In general in LedgerSMB we use both, depending on what kind of search we are doing. You might see a search like:
select * from parts
WHERE partnumber ilike ? || '%'
and plainto_tsquery(get_default_language(), ?) ## description;
So these are very different. Use each one where it makes the most sense.
I have two columns say Main and Sub. (they can be of same table or not).
Main is varchar of length 20 and Sub is varchar of length 8.
Sub is always subset of Main and it is last 8 characters of Main.
I could successfully design a query to match pattern using substr("Main",13,8)
Query:
select * from "MainTable"
where substr("MainColumn",13,8) LIKE (
select "SubColumn" From "SubTable" Where "SubId"=1043);
but I want to use Like, % , _ etc in my query so that I can loosely match the pattern (that is not all 8 characters).
Question is how can i do that.?!
I know that the query below is COMPLETELY WRONG but I want to achieve something like this,
Select * from "MainTable"
Where "MainColumn" Like '%' Select "SubColumn" From "SubTable" Where "SubId"=2'
The answers so far fail to address your question:
but I want use Like, % , _ etc in my query so that I can loosely match
the pattern (that is not all 8 characters).
It makes hardly any difference whether you use LIKE or = as long as you match the whole string (and there are no wildcard character in your string). To make the search fuzzy, you need to replace part of the pattern, not just add to it.
For instance, to match on the last 7 (instead of 8) characters of subcolumn:
SELECT *
FROM maintable m
WHERE left(maincolumn, 8) LIKE
( '%' || left((SELECT subcolumn FROM subtable WHERE subid = 2), 7));
I use the simpler left() (introduced with Postgres 9.1).
You could simplify this to:
SELECT *
FROM maintable m
WHERE left(maincolumn, 7) =
(SELECT left(subcolumn,7) FROM subtable WHERE subid = 2);
But you wouldn't if you use the special index I mention further down, because expressions in functional indexes have to matched precisely to be of use.
You may be interested in the extension pg_tgrm.
In PostgreSQL 9.1 run once per database:
CREATE EXTENSION pg_tgrm;
Two reasons:
It supplies the similarity operator %. With it you can build a smart similarity search:
--SELECT show_limit();
SELECT set_limit(0.5); -- adjust similarity limit for % operator
SELECT *
FROM maintable m
WHERE left(maincolumn, 8) %
(SELECT subcolumn FROM subtable WHERE subid = 2);
It supplies index support for both LIKE and %
If read performance is more important than write performance, I suggest you create a functional GIN or GiST index like this:
CREATE INDEX maintable_maincol_tgrm_idx ON maintable
USING gist (left(maincolumn, 8) gist_trgm_ops);
This index supports either query. Be aware that it comes with some cost for write operations.
A quick benchmark for a similar case in this related answer.
Try
SELECT t1.* from "Main Table" AS t1, "SubTable" AS t2
WHERE t2.SubId=1043
AND substr(t1.MainColumn, 13, 8) LIKE "%" || CAST(t2.SubColumn as text);
Argument to a LIKE is an ordinary string, so all string manipulations are valid here.
In your case you need to concatenate wildchars with the target substring, like #bksi suggests:
... LIKE '%'||CAST("SubColumn" AS test) ...
Note, though, that such patterns (the ones starting with a % wildcard) are badly performing ones. Take a look at PostgreSQL LIKE query performance variations.
I would recommend:
sticking with the current substr("MainColumn", 13, 8) approach;
avoid LIKE and use equality comparison (=) instead (although they're equal if LIKE pattern contains no wildcards, it is easier to read the query);
build an expression index on the "MainTable" the following way:
CREATE INDEX i_maincolumn ON "MainTable" (substr("MainColumn", 13, 8));
This combination will perform better in my view.
And use lowercase names for the tables/columns, so that you can avoid doublequoting them.