postgres column alias problem

Question

As a newbie to Postgresql (I\'m moving over because I\'m moving my site to heroku who only support it, I\'m having to refactor some of my queries and code. Here\'s a problem

Answer 1

I ran into this same problem using functions from fuzzystrmatch - particularly the levenshtein function. I needed to both sort by the string distance, and filter results by the string distance. I was originally trying:

SELECT thing.*, 
levenshtein(thing.name, '%s') AS dist 
FROM thing 
WHERE dist < character_length(thing.name)/2 
ORDER BY dist

But, of course, I got the error "column"dist" does not exist" from the WHERE clause. I tried this and it worked:

SELECT thing.*, 
(levenshtein(thing.name, '%s')) AS dist 
FROM thing 
ORDER BY dist

But I needed to have that qualification in the WHERE clause. Someone else in this question said that the WHERE clause is evaluated before ORDER BY, thus the column was non-existent when it evaluated the WHERE clause. Going by that advice, I figured out that a nested SELECT statement does the trick:

SELECT * FROM 
(SELECT thing.*, 
     (levenshtein(thing.name, '%s')) AS dist 
     FROM thing 
     ORDER BY dist
) items 
WHERE dist < (character_length(items.name)/2)

Note that the "items" table alias is required and the dist column alias is accessible in the outer SELECT because it's unique in the statement. It's a little bit funky and I'm surprised that it has to be this way in PG - but it doesn't seem to take a performance hit so I'm satisfied.