insensitive indexes on Postgres string array

I'm using a varchar[] column (varchar array) in Postgres 9.2 to store some tags. While retrieving rows by tags, I want the query to be case insensitive. However, I want to preserve the case to display in the UI (therefore I can't just store everything as lower case).

So, my question is how do I create a case-insensitive index in Postgres over a varchar array? One possible approach would be to create a functional GIN index on the column. How does one do that? Any other approaches?

---

@Saurabh Nanda: Similar to what you posted, you can also create a simple function to convert your varchar array to lowercase as follows:

CREATE OR REPLACE FUNCTION array_lowercase(varchar[]) RETURNS varchar[] AS
$BODY$
  SELECT array_agg(q.tag) FROM (
    SELECT btrim(lower(unnest($1)))::varchar AS tag
  ) AS q;
$BODY$
  language sql IMMUTABLE;

Note that I'm also trimming the tags of spaces. This might not be necessary for you but I usually do for consistency.

Testing:

SELECT array_lowercase(array['Hello','WOrLD']);
 array_lowercase 
-----------------
 {hello,world}
(1 row)

As noted by Saurabh, you can then create a GIN index:

CREATE INDEX ix_tags ON tagtable USING GIN(array_lowercase(tags));

And query:

SELECT * FROM tagtable WHERE ARRAY['mytag'::varchar] && array_lowercase(tags);

UPDATE: Performance of WHILE vs array_agg/unnest

I created table of 100K 10 element text[] arrays (12 character random mixed case strings) and tested each function.

The array_agg/unnest function returned:

EXPLAIN ANALYZE VERBOSE SELECT array_lowercase(data) FROM test;
                                                       QUERY PLAN                                                       
------------------------------------------------------------------------------------------------------------------------
 Seq Scan on public.test  (cost=0.00..28703.00 rows=100000 width=184) (actual time=0.320..3041.292 rows=100000 loops=1)
   Output: array_lowercase((data)::character varying[])
 Total runtime: 3174.690 ms
(3 rows)

The WHILE function returned:

EXPLAIN ANALYZE VERBOSE SELECT array_lowercase_while(data) FROM test;
                                                       QUERY PLAN                                                       
------------------------------------------------------------------------------------------------------------------------
 Seq Scan on public.test  (cost=0.00..28703.00 rows=100000 width=184) (actual time=5.128..4356.647 rows=100000 loops=1)
   Output: array_lowercase_while((data)::character varying[])
 Total runtime: 4485.226 ms
(3 rows)

UPDATE 2: FOREACH vs. WHILE As a final experiment, I changed the WHILE function to use FOREACH:

CREATE OR REPLACE FUNCTION array_lowercase_foreach(p_input varchar[]) RETURNS varchar[] AS $BODY$
DECLARE
    el text;
    r varchar[];
BEGIN
    FOREACH el IN ARRAY p_input LOOP
        r := r || btrim(lower(el))::varchar;
    END LOOP;
    RETURN r;
END;
$BODY$
  language 'plpgsql'

Results appeared to be similar to WHILE :

EXPLAIN ANALYZE VERBOSE SELECT array_lowercase_foreach(data) FROM test;
                                                       QUERY PLAN                                                       
------------------------------------------------------------------------------------------------------------------------
 Seq Scan on public.test  (cost=0.00..28703.00 rows=100000 width=184) (actual time=0.707..4106.867 rows=100000 loops=1)
   Output: array_lowercase_foreach((data)::character varying[])
 Total runtime: 4239.958 ms
(3 rows)

Though my tests are not by any means rigorous, I did run each version a number of times and found the numbers to be representative, suggesting that the SQL method (array_agg/unnest) is the fastest.

---

Found one possible approach using a custom pgplsql function:

First declare a custom function that takes a varchar[] array as input and returns a new array with all elements converted to lowercase. (This is the first time I'm writing PL/SQL, so this might be very inefficient code).

CREATE OR REPLACE FUNCTION array_lowercase(varchar[]) RETURNS varchar[] AS $$
DECLARE
    i INTEGER;
    l INTEGER;
    r VARCHAR[];
    inp ALIAS FOR $1;
BEGIN
    i := 1;
    l := array_length($1, 1);
    WHILE i <= l LOOP
        r[i] = lower(inp[i]);
        i := i + 1;
    END LOOP;
    RETURN r;
END;
$$ LANGUAGE plpgsql IMMUTABLE;

Next, create a GIN index on the expression using the newly defined array_lowercase function:

create index hotel_bookings_tags on hotel_bookings using gin(array_lowercase(tags));

Now use it in a query (verify that it's using the index using EXPLAIN):

select * from posts where array[(varchar 'some_tag')] && array_lowercase(tags);

---

不知道它有帮助，但我正在寻找类似于text[]东西，并使用了类型转换：

select id from product where lower(tags::text)::text[] && array['tat'];

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