postgresql不在text列上使用trigram索引,但在varchar列上使用它

以下是表和索引定义：

-- Table: fulltextproba
-- DROP TABLE fulltextproba;
CREATE TABLE fulltextproba
(
  id integer NOT NULL,text text,varchar600 character varying(600) COLLATE pg_catalog."C.UTF-8",CONSTRAINT id PRIMARY KEY (id )
)
WITH (
  OIDS=FALSE
);

-- Index: id_index
-- DROP INDEX id_index;
CREATE UNIQUE INDEX id_index
  ON fulltextproba
  USING btree
  (id );

-- Index: text_gin_fulltext_hun
-- DROP INDEX text_gin_fulltext_hun;
CREATE INDEX text_gin_fulltext_hun
  ON fulltextproba
  USING gin
  (to_tsvector('hungarian'::text,text) );

-- Index: text_gin_trgm
-- DROP INDEX text_gin_trgm;
CREATE INDEX text_gin_trgm
  ON fulltextproba
  USING gin
  (text COLLATE pg_catalog."C.UTF-8" gin_trgm_ops);

-- Index: varchar600
-- DROP INDEX varchar600;
CREATE INDEX varchar600
  ON fulltextproba
  USING btree
  (varchar600 COLLATE pg_catalog."C.UTF-8" varchar_pattern_ops);

-- Index: varchar600_gin_trgm
-- DROP INDEX varchar600_gin_trgm;
CREATE INDEX varchar600_gin_trgm
  ON fulltextproba
  USING gin
  (varchar600 COLLATE pg_catalog."C.UTF-8" gin_trgm_ops);

我的问题是,如果我执行％foo％搜索应该使用trigram索引,如果我搜索文本列,它不会：

SELECT COUNT(id) FROM public.fulltextproba WHERE text LIKE '%almáv%'
 count 
-------
   396
(1 row)

real    0m7.215s
user    0m0.020s
sys 0m0.004s
                                QUERY PLAN                                 
---------------------------------------------------------------------------
 Aggregate  (cost=657056.11..657056.12 rows=1 width=4)
   ->  Seq Scan on fulltextproba  (cost=0.00..657052.72 rows=1355 width=4)
         Filter: (text ~~ '%almáv%'::text)
(3 rows)

但是,如果我在varchar600列中搜索,它确实使用了trigram索引,并且 – 并不令人惊讶 – 更快：

SELECT COUNT(id) FROM public.fulltextproba WHERE varchar600 LIKE '%almáv%'
 count 
-------
   373
(1 row)

real    0m0.184s
user    0m0.052s
sys 0m0.004s
                                         QUERY PLAN                                         
--------------------------------------------------------------------------------------------
 Aggregate  (cost=5283.11..5283.12 rows=1 width=4)
   ->  Bitmap Heap Scan on fulltextproba  (cost=62.50..5279.73 rows=1355 width=4)
         Recheck Cond: ((varchar600)::text ~~ '%almáv%'::text)
         ->  Bitmap Index Scan on varchar600_gin_trgm  (cost=0.00..62.16 rows=1355 width=0)
               Index Cond: ((varchar600)::text ~~ '%almáv%'::text)
(5 rows)

所以最终的问题是：

>为什么postgres不在文本列上使用trigram索引.
>如何使用postgres来使用索引？我应该用其他方式来定义吗？