Index Usage and Running Times by FullTextSearch with prefix matching

Hi

I tested the following:

CREATE TABLE t1
(
id serial NOT NULL,
a character varying(125),
a_tsvector tsvector,
CONSTRAINT t1_pkey PRIMARY KEY (id)
);

INSERT INTO t1 (a, a_tsvector)
VALUES ('ooooo,ppppp,fffff,jjjjj,zzzzz,jjjjj',
to_tsvector('ooooo,ppppp,fffff,jjjjj,zzzzz,jjjjj');

CREATE INDEX a_tsvector_idx ON t1 USING gin (a_tsvector);

(I have generated 900000 records with random words like this)

Now querying: normal full text search

SELECT count(a)
FROM t1
WHERE a_tsvector @@ to_tsquery('aaaaa & bbbbb & ccccc & ddddd')

(RESULT: count: 619)
Total query runtime: 353 ms.
Query Plan:
"Aggregate (cost=6315.22..6315.23 rows=1 width=36)"
" -> Bitmap Heap Scan on t1 (cost=811.66..6311.46 rows=1504 width=36)"
" Recheck Cond: (a_tsvector @@ to_tsquery('aaaaa & bbbbb & ccccc &
ddddd'::text))"
" -> Bitmap Index Scan on a_tsvector_idx (cost=0.00..811.28
rows=1504 width=0)"
" Index Cond: (a_tsvector @@ to_tsquery('aaaaa & bbbbb & ccccc
& ddddd'::text))"

And querying: FTS with prefix matching:

SELECT count(a)
FROM t1
WHERE a_tsvector @@ to_tsquery('aaa:* & b:* & c:* & d:*')

(RESULT: count: 619)
Total query runtime: 21266 ms.
Query Plan:
"Aggregate (cost=804.02..804.03 rows=1 width=36)"
" -> Bitmap Heap Scan on t1 (cost=800.00..804.02 rows=1 width=36)"
" Recheck Cond: (a_tsvector @@ to_tsquery('aaa:* & b:* & c:* &
d:*'::text))"
" -> Bitmap Index Scan on a_tsvector_idx (cost=0.00..800.00 rows=1
width=0)"
" Index Cond: (a_tsvector @@ to_tsquery('aaa:* & b:* & c:* &
d:*'::text))"

I don't understand the big query time difference, despite the explainig
index usage.
NOnetheless I'd like to simulate LIKE 'aaa%' with full text search. Would I
have a better sollution?

Many thanks in advance!

Rawi

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