I have a Postgresql 11 database. Let's say I have a table called houses. It should have hundreds of thousands of records.
CREATE TABLE houses ( pkid serial primary key, address varchar(255) NOT NULL, rent float NOT NULL );
Now, my houses have features I want to register in the database. As the list of possible features will be quite long (several dozens) and will evolve over time, as I don't want add a long list of columns to the table houses and change the table constantly with 'ALTER TABLE', I thought of having a separate table for these features :
CREATE TABLE house_features ( pkid serial primary key, house_pkid integer NOT NULL, feature_name varchar(255) NOT NULL, feature_value varchar(255) ); CREATE INDEX ON house_features (feature_name, feature_value); ALTER TABLE house_features ADD CONSTRAINT features_fk FOREIGN KEY (house_pkid) REFERENCES houses (pkid) ON DELETE CASCADE;
In average, each house record will have 10-20 records in the house_features table.
So far, this seems a simple efficient model : I can add as many different features, controlling the possible values of feature_name and feature_value in the upper layers (the applicative layer and/or the GUI). I don't have to alter the database each time the application evolves and I need a new type of feature.
For the example, let's say I have the following features :
- feature_name : 'rooftype' with possible feature_value : 'flat' or 'inclined'
- feature_name : 'wallcolors' with possible feature_value : 'white', 'beige', 'blue', 'green', etc.. (15 different possible values)
- feature_name : 'has_basement' with possible feature_value : 'True' or 'False'.
- feature_name : 'number_of_doors' with possible feature_value any integer coded as a string (so '0', '1', '2', ...).
- feature_name : 'floor_surface' with possible feature_value any given float coded as a string (e.g.: '155.2')
Obviously, storing booleans, integers and floats as strings is not very efficient and this is also something I will need to take care of. I was thinking of having a separate house_features_XXX table for each XXX type (string, boolean, float, integer).
But that is not even my problem.
My problem is : how do I search for houses that have certain features ?
For the example, let's say I want to search the houses with a basement, white walls and an inclined rooftype. I could dynamically create in the application layer a query like :
SELECT sq1.* FROM ( SELECT house_pkid FROM house_features WHERE feature_name = 'has_basement' AND feature_value = 'True' ) AS sq1 JOIN ( SELECT house_pkid FROM house_features WHERE feature_name = 'wallcolors' AND feature_value = 'white' ) AS sq2 ON sq1.house_pkid = sq2.house_pkid JOIN ( SELECT house_pkid FROM house_features WHERE feature_name = 'rooftype' AND feature_value = 'inclined' ) AS sq3 ON sq1.house_pkid = sq3.house_pkid ;
But that seems not so efficient, especially considering that there may be several dozens of conditions on house_features.
Is there a better way to do this ?