Hackers,
WIP patch with index support for regexp search for pg_trgm contrib is attached.
In spite of techniques which extracts continuous text parts from regexp, this patch presents technique of automatum transformation. That allows more comprehensive trigrams extraction.
A little example of possible perfomance benefit.
test=# explain analyze select * from words where s ~ 'a[bc]+[de]';
QUERY PLAN
----------------------------------------------------------------------------------------------------
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Seq Scan on words (cost=0.00..1703.11 rows=10 width=9) (actual time=3.092..242.303 rows=662 loops=1)
Filter: (s ~ 'a[bc]+[de]'::text)
Rows Removed by Filter: 97907
Total runtime: 243.213 ms
(4 rows)
test=# explain analyze select * from words where s ~ 'a[bc]+[de]';
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on words (cost=260.08..295.83 rows=10 width=9) (actual time=4.166..7.506 rows=662 loops=1)
Recheck Cond: (s ~ 'a[bc]+[de]'::text)
Rows Removed by Index Recheck: 18
-> Bitmap Index Scan on words_trgm_idx (cost=0.00..260.07 rows=10 width=0) (actual time=4.076..4.076 rows=680 loops=1)
Index Cond: (s ~ 'a[bc]+[de]'::text)
Total runtime: 8.424 ms
(6 rows)
Current version of patch have some limitations:
1) Algorithm of logical expression extraction on trigrams have high computational complexity. So, it can become really slow on regexp with many branches. Probably, improvements of this algorithm is possible.
2) Surely, no perfomance benefit if no trigrams can be extracted from regexp. It's inevitably.
3) Currently, only GIN index is supported. There are no serious problems, GiST code for it just not written yet.
4) It appear to be some kind of problem to extract multibyte encoded character from pg_wchar. I've posted question about it here:
While I've hardcoded some dirty solution. So PG_EUC_JP, PG_EUC_CN, PG_EUC_KR, PG_EUC_TW, PG_EUC_JIS_2004 are not supported yet.
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With best regards,
Alexander Korotkov.
