Index on column with data type citext not used In PostgreSQL 9.4, with following schema:

CREATE TABLE people (
    id INTEGER PRIMARY KEY,
    name TEXT,
    junk CHAR(1000)
);

INSERT INTO people(id, name)
 SELECT generate_series(1,100000), md5(random()::text);

CREATE INDEX ON people (name text_pattern_ops);


if I search by name, the index is used:

test=# explain analyze select id, name from people where name like 'a%';
                                                           QUERY PLAN                                                           
--------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on people  (cost=248.59..1160.92 rows=6061 width=37) (actual time=2.412..8.340 rows=6271 loops=1)
   Filter: (name ~~ 'a%'::text)
   Heap Blocks: exact=834
   -&gt;  Bitmap Index Scan on people_name_idx  (cost=0.00..247.08 rows=6266 width=0) (actual time=2.123..2.123 rows=6271 loops=1)
         Index Cond: ((name ~&gt;=~ 'a'::text) AND (name ~&lt;~ 'b'::text))
 Planning time: 0.600 ms
 Execution time: 8.991 ms


But if I replace TEXT with CITEXT:

CREATE EXTENSION CIText;

CREATE TABLE people (
    id INTEGER PRIMARY KEY,
    name CITEXT,
    junk CHAR(1000)
);


The index is not used anymore:

test=# explain analyze select id, name from people where name like 'a%';
                                                QUERY PLAN                                                 
-----------------------------------------------------------------------------------------------------------
 Seq Scan on people  (cost=0.00..2084.00 rows=500 width=36) (actual time=5.700..152.572 rows=6305 loops=1)
   Filter: (name ~~ 'a%'::citext)
   Rows Removed by Filter: 93695
 Planning time: 0.764 ms
 Execution time: 153.046 ms


According to CITEXT PostgreSQL documentation, the behavior should be as with TEXT: 


  Otherwise, it behaves almost exactly like text.


How can I tell PostgreSQL to use the index?
Index usage with text_pattern_ops (as well as with the default operator class when using the C locale) depends on the binary representation of character data. citext stores original values with the case preserved, so there must be a problem with that ...

Like you commented, the actual reason is burried in collation support.

Either way, citext or text, you can make it work with an expression index:

CREATE INDEX people_name_idx ON people (lower(name) text_pattern_ops);


And a corresponding query:

SELECT id, name FROM people WHERE lower(name) LIKE 'abc%';


Note that lower(name) returns data type text, even when feeding it citext.

Alternatively, you could use a trigram index, which is more expensive to maintain, but also offers more capabilities:


How is LIKE implemented?
Pattern matching with LIKE, SIMILAR TO or regular expressions in PostgreSQL


Aside: your test case is suboptimal because your dummy values are all lower case to begin with and the pattern 'a%' is often not selective enough to use an index at all. And char(1000) doesn't make sense (even though irrelevant to the test).