Is it possible to create an index of a table based on substring of one or more field values in DB2 ?
The docs imply that you can create an index on a "key-expression" rather than just a vanilla column see key-expression here.
Related
My table (table) has a JSONB field (data) that contains a field with an array where I store tags (tags).
I query that table with an expression like:
SELECT * FROM table WHERE data->'tags' #? '$[*] ? (# like_regex ".*(foo|bar).*" flag "i");
With such use-case is there a way for me to index the data->'tags' array to speed up the query? Or should I rather work on moving the tags array out of the JSONB field and into a TEXT[] field and index that?
I've already tried:
CREATE INDEX foo ON tbl USING GIN ((data->'tags') jsonb_path_ops);
but it doesn't work: https://gist.github.com/vkaracic/a62ac917d34eb6e975c4daeefbd316e8
The index you built can be used (if you set enable_seqscan=off, you will see that it does get used), but it is generally not chosen as it is pretty useless for this query. The only rows it would rule out through the index are the ones that don't have the 'tags' key at all, and even at that is poorly estimated so probably won't be used without drastic measures.
You could try to convert to text[] and the use parray_gin, but probably better would be to convert to a child table with text and then use pg_trgm.
This might be an obvious and simple question.
But I read through the jsonb data type documentation, but nowhere it mentions the lookup cost of a key in jsonb data.
For example, let's say I have a table with following schema:
CREATE TABLE A (id character varying (20),
info jsonb);
I want to know how postgres would parse a where query as below:
SELECT * FROM A WHERE info->>'city' = 'portland';
While going through the jsonb field of a row, is the lookup constant time (O(1)) or linear time (checking each key one by one in the row's jsonb dictionary) within that jsonb data dictionary?
My intuition is that it must be constant time (else what's the point of a dictionary style data?) but I can't see it in the official documentation to convince my team.
Any help would be great!
Thanks!
As with any WHERE condition in SQL: if there is no index, the database has to go through all rows of the table to find those that satisfy your condition.
You can either index a specific expression, or you can index the whole json value using a GIN index which then enables Postgres to use the index if any of the supported operators are used.
If you always check for the city, you can create a regular B-Tree index:
create index on a ( (info->>'city') );
If you don't know what you will be looking for, a GIN index might be a better choice:
create index on a using gin (info);
But you will need to change your query to use one of the operators that are supported by a GIN index, e.g. using the contains operator #>
select *
from a
where info #> '{"city": "portland"}::jsonb;
Note that an index lookup is not always the most efficient solution. Sometimes it's faster to simply go through all rows, sometimes the index lookup is faster.
If you want to learn more about indexes in relational database, go through the material here: http://use-the-index-luke.com/
I have to write a migration command to remove a column from the index. Currently let us say I have table1 that has index on col1 and col2
I want to remove col1 from the index. I am looking at https://www.postgresql.org/docs/9.4/static/sql-alterindex.html but it does not seem I can actually just remove a column?
If yes, will it be better to remove the column and how VS
Create new Index
Drop the old index
Also, I want to do the reverse if I need to do downgrade. So just wondering how to achieve this
The ability to alter an index doesn't exist because in order to do so you would have to destroy and recreate the index with the new columns. By default, Postgres uses B-Trees to create indices and removing a column causes that B-Tree to become invalid. As a result the B-Tree needs to be built from scratch.
If you want some more details on how indices work under the hood, this is a good article: Postgres Indices Under the Hood
You’re right, you’ll have to create a new index with a single column and then drop an old index with two columns.
I have question about how postgresql use indexes. I have problems with Gist Index Expression based on Geography Type Column in Postgresql with Postgis enabled database.
I have the following table:
CREATE TABLE place
(
id serial NOT NULL,
name character varying(40) NOT NULL,
location geography(Point,4326),
CONSTRAINT place_pkey PRIMARY KEY (id )
)
Then I created Gist Index Expression based on column "location"
CREATE INDEX place_buffer_5000m ON place
USING GIST (ST_BUFFER(location, 5000));
Now suppose that in table route I have column shape with Linestring object and I want to check which 5000m polygons (around the location) the line crosses.
The query below in my opinion shoud use the "place_buffer_5000m" index but does not use it.
SELECT place.name
FROM place, route
WHERE
route.id=1 AND
ST_CROSSES(route.shape::geometry, ST_BUFFER(place.location, 5000)::geometry))
Table place have about 76000 rows. Analyze and Vacuum was run on this table with recreating "place_buffer_5000m" index but the index is not used during the above query.
What is funny when I create another column in table place named "area_5000m" (geograpthy type) and update the table like:
UPDATE place SET area_5000m=ST_BUFFER(location, 5000)
And then create gist index for this column like this:
CREATE INDEX place_area_5000m ON place USING GIST (area_5000m)
Then using the query:
SELECT place.name
FROM place, route
WHERE
route.id=1 AND
ST_CROSSES(route.shape::geometry, place.area_5000m::geometry))
The index "place_area_5000m" is used.
The question is why the Index expression that is calculated based on location column is not used?
Did you try to add a cast to your "functional index"?
This could help to determine the data type.
It should work with geometry and probably also for geography, like this:
CREATE INDEX place_buffer_5000m ON place
USING GIST(ST_BUFFER(location, 5000)::geometry);
Ultimately, you want to know what routes are within 5 km of places, which is a really simple and common type of query. However, you are falling into a common trap: don't use ST_Buffer to filter! It is expensive!
Use ST_DWithin, which will use a regular GiST index (if available):
SELECT place.name
FROM place, route
WHERE route.id = 1 AND ST_DWithin(route.shape::geography, place.location, 5000);
I am a newbie in postgres. I have a column named host (string varchar2) in a table which has around 20 million rows. How do I use indexing to optimize my search to find particular host. Also, this column will be updated daily do I need to write trigger indexing at particular interval? If yes, how do I do that? (For Records I am using Ruby and Rails 3)
Assuming you're doing exact matches, you should just be able to create the index and leave it:
CREATE INDEX host_index ON table_name (host)
The query optimizer should just use that automatically.
You may wish to specify other options such as the collation to use.
See the PostgreSQL docs for CREATE INDEX for more information.
I'd suggest using BRIN Index since its introduction from PostgreSQL 9.5 rather than the conventional btree index.
For text search, it is recommended that you use GIN or GiST index types.
https://www.postgresql.org/docs/9.5/static/textsearch-indexes.html
Another possibility is that if you were only performing exact matching in the host column, i.e., no inequality comparisons (>, <) and partial matching (like, wildcard) involved, you may consider converting host to a hash integer to speed up the search significantly.