Thread: Planner does not use btree index for LIKE 'prefix%' on text column, but does for equivalent range query (PostgreSQL 17.4)

Planner does not use btree index for LIKE 'prefix%' on text column, but does for equivalent range query (PostgreSQL 17.4)

From

Abdullah DURSUN

Date:

03 May, 19:50:00

Hello PostgreSQL team,

I have encountered what appears to be a planner regression or bug in PostgreSQL 17.4 regarding the use of btree indexes for prefix searches with LIKE 'prefix%'.

A btree index on a text column is not used for a query of the form WHERE col LIKE 'prefix%', even though the index is healthy, statistics are correct, and the query is highly selective.

The same index is used and performs well for an equivalent range query (WHERE col >= 'prefix' AND col < 'nextprefix').

- This behavior persists even after VACUUM FULL, ANALYZE, and index re-creation.
- The column and index use the default collation (en_US.UTF-8).
- The index is valid and ready.
- The planner does not use the index for LIKE 'prefix%', but does for the equivalent range.
- Rewriting the query as a range (col >= 'prefix' AND col < 'nextprefix') uses the index and is fast, but this should not be necessary.

Thank you for your attention!

Re: Planner does not use btree index for LIKE 'prefix%' on text column, but does for equivalent range query (PostgreSQL 17.4)

From

Tom Lane

Date:

03 May, 23:04:14

Abdullah DURSUN <adursuns@gmail.com> writes:
> A btree index on a text column is not used for a query of the form WHERE
> col LIKE 'prefix%', even though the index is healthy, statistics are
> correct, and the query is highly selective.

LIKE with a prefix condition can only use an index if the index's
collation is "C", or if it uses the "pattern" opclass.  This isn't
new in v17, it's been true for a very long time.

d1=# create table foo (t text);
CREATE TABLE
d1=# create index on foo (t);
CREATE INDEX
d1=# explain select * from foo where t like 'prefix%';
                     QUERY PLAN                      
-----------------------------------------------------
 Seq Scan on foo  (cost=0.00..27.00 rows=7 width=32)
   Filter: (t ~~ 'prefix%'::text)
(2 rows)

d1=# create index on foo (t collate "C");
CREATE INDEX
d1=# explain select * from foo where t like 'prefix%';
                               QUERY PLAN                                
-------------------------------------------------------------------------
 Bitmap Heap Scan on foo  (cost=4.22..14.37 rows=7 width=32)
   Filter: (t ~~ 'prefix%'::text)
   ->  Bitmap Index Scan on foo_t_idx1  (cost=0.00..4.22 rows=7 width=0)
         Index Cond: ((t >= 'prefix'::text) AND (t < 'prefiy'::text))
(4 rows)

            regards, tom lane