How do I preserve the order of the original elements in an unnested array?

Question

Given the string

'I think that postgresql is nifty'

, I would like to operate on the individual words found within that string. Essentially, I have a separate from which I can get word details and would like to join an unnested array of that string on this dictionary.

So far I have

select word, meaning, partofspeech
from unnest(string_to_array('I think that postgresql is nifty',' ')) as word
from table t
join dictionary d
on t.word = d.wordname;

This accomplishes the fundamentals of what I was hoping to do, but it does not preserve the original word order.

Related Question: Postgresql unnest() with element number

Answer 1

For a single string

You can apply the window function row_number() to remember a distinct order of elements. However, with the usual row_number() OVER (ORDER BY col) you get numbers according to the sort order, not the ordinal number of the original position in the string.

You could try and simply omit the ORDER BY to get the position "as is":

SELECT *, row_number() OVER () AS rn
FROM  (
   SELECT regexp_split_to_table('I think postgres is nifty', ' ') AS word
   ) x;

Performance of regexp_split_to_table() degrades with long strings. unnest(string_to_array(...)) scales better:

SELECT *, row_number() OVER () AS rn
FROM  (
   SELECT unnest(string_to_array('I think postgres is nifty', ' ')) AS word
   ) x;

However, while this works normally, and I have never seen it break in simple queries, PostgreSQL asserts nothing concerning the order of rows without explicit ORDER BY.

To guarantee row numbers matching the ordinal number of the element in the string:

SELECT arr[rn] AS word, rn
FROM   (
   SELECT *, generate_subscripts(arr, 1) AS rn
   FROM  (
      SELECT string_to_array('I think postgres is nifty', ' ') AS arr
      ) x
   ) y;

http://www.postgresql.org/docs/current/interactive/functions-srf.html#FUNCTIONS-SRF-SUBSCRIPTS

For a table of strings

Add PARTITION BY id to the OVER clause ..

Demo table:

CREATE TEMP TABLE strings(string text);
INSERT INTO strings VALUES
  ('I think postgres is nifty')
 ,('And it keeps getting better');

I use ctid as ad-hoc pk substitute. If you have a primary key or any unique column available use that instead.

SELECT *, row_number() OVER (PARTITION BY ctid) AS rn
FROM  (
   SELECT ctid, unnest(string_to_array(string, ' ')) AS word
   FROM   strings
   ) x;

This works without any distinct ID:

SELECT arr[rn] AS word, rn
FROM  (
   SELECT *, generate_subscripts(arr, 1) AS rn
   FROM  (
      SELECT string_to_array(string, ' ') AS arr
      FROM   strings
      ) x
   ) y

-> sqlfiddle.

Answer to question

SELECT z.arr, z.rn, z.word, d.meaning   -- , partofspeech --?
FROM  (
   SELECT *, arr[rn] AS word
   FROM   (
       SELECT *, generate_subscripts(arr, 1) AS rn
       FROM  (
         SELECT string_to_array(string, ' ') AS arr
         FROM   strings
         ) x
      ) y
   ) z
JOIN   dictionary d ON d.wordname = z.word
ORDER  BY z.arr, z.rn;