Why postgresql does not use index in this query

问题

Here is the complete table definition:

CREATE TABLE search.tablename
(
  id integer NOT NULL,
  name character varying(300) NOT NULL,
  CONSTRAINT tablename_pkey PRIMARY KEY (id)
)
WITH (
  OIDS=FALSE
);

CREATE INDEX tablename_name_idx
  ON search.tablename
  USING btree
  (name COLLATE pg_catalog."default");

It has ~73k rows and was just vacuumed/analyzed by manual run.

What I cannot understand is why this trivial query

SELECT * 
  FROM "tablename" AS "arn" 
 WHERE arn.name LIKE 'foo%'

does not use the corresponding index.

The execution plan for this query is

Seq Scan on tablename arn  (cost=0.00..1626.96 rows=7 width=47) (actual time=8.682..8.682 rows=0 loops=1)
  Filter: ((name)::text ~~ 'foo%'::text)
  Rows Removed by Filter: 73197
Total runtime: 8.703 ms

Could anyone point to what I am missing?

UPD: changing column name type to text changes nothing.

UPD 2: name = 'foo' predicate expectedly uses index

回答1:

Try adding varchar_pattern_ops to your index

CREATE INDEX tablename_name_idx
  ON search.tablename
  USING btree
  (name COLLATE pg_catalog."default" varchar_pattern_ops);

The operator classes text_pattern_ops, varchar_pattern_ops, bpchar_pattern_ops, and name_pattern_ops support B-tree indexes on the types text, varchar, char, and name, respectively. The difference from the default operator classes is that the values are compared strictly character by character rather than according to the locale-specific collation rules. This makes these operator classes suitable for use by queries involving pattern matching expressions (LIKE or POSIX regular expressions) when the server does not use the standard "C" locale.

来源：https://stackoverflow.com/questions/27435980/why-postgresql-does-not-use-index-in-this-query

标签

sql

postgresql

postgresql-9.3