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Let's say I have a table representing principal country divisions (ex States):

create table principal_country_divisions (
  id int primary key,
  name text not null,
  country_code char(2)
);

insert into principal_country_divisions values (1, 'New York', 'US');

I want users to be easily able to find New York via synonyms such as 'New York', 'NY', or 'New York State'.

So I have a synonyms table:

create table synonyms (
  syn text,
  name text,

  primary key (syn, name)
);

insert into synonyms values 
('NY', 'New York'),
('New York State', 'New York');

What is an efficient and easy way to query this and return ONE record for New York?

In particular, they should be able to find the result for the default name 'New York' OR any synonym:

select * from principal_country_divisions where name = 'NY';
result: {1, 'New York', 'US'}

I guess I would start with something like this:

select
id,
name,
country_code
from principal_country_divisions a
where name = 'NY'
or exists (select 1 from synonyms where name = a.name and syn = 'NY')

Can I do this only using a view, or should I use a function?

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1 Answer 1

First of all, you have an integer primary key on principal_country_divisions. Use it. More efficient than joining via name for multiple reasons (storage size, index size, faster integer arithmetic, no collations involved, fixed length).

create table principal_country_divisions (
  country_id    int primary key
  ,name         text not null
  ,country_code char(2)
);

create table synonyms (
   country_id int REFERENCES principal_country_divisions (country_id)
  ,syn        text
  ,primary    key (syn, country_id)
);

syn needs to the the first column of the index (pk), you had that right already. The accompanying index automatically covers equality tests on synonyms.syn.
Be sure to add an index on principal_country_divisions.name:

CREATE INDEX foo ON principal_country_divisions (name);

If you'd want to match patterns, not whole strings, the job would become more complex.

Next, how can you be sure to

return ONE record for New York?

Obviously, name and syn can be the same. There is no unique constraint over both columns and there isn't even one on syn alone. Otherwise your EXISTS query is a good approach - usually fast. You'd just have to avoid multiple rows. The added benefit of EXISTS would be to eliminate duplicates from synonyms alone, but that's ruled out by the pk. This may be faster for the case:

SELECT DISTINCT ON (1)
       a.country_id, a.name, a.country_code
FROM   principal_country_divisions a
LEFT   JOIN synonyms               s USING (country_id)
WHERE  a.name = 'NY'
OR     s.syn  = 'NY'
-- ORDER BY 1, <more expressions to pick from peers>

As you commented, a LEFT JOIN is in order to preserve finds in name.

In case of multiple finds, you can chose what to pick by adding more ORDER BY expressions. Leading columns have to agree with DISTINCT ON, though. Details in this related answer on SO.

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1  
Thanks Erwin. Looks like your last query won't work if a country doesn't have a synonym. A left join seems to fix it. –  Neil McGuigan Dec 17 '13 at 21:11
    
@NeilMcGuigan: Exactly, that was an oversight on my part. Fixed and added a bit more. –  Erwin Brandstetter Dec 18 '13 at 0:03

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