Re: Avoid full GIN index scan when possible

Hi!

On 29.06.2019 1:23, Julien Rouhaud wrote:
>>> But that kinda resembles stuff we already have - selectivity/cost. So
>>> why shouldn't this be considered as part of costing?
>> Yeah, I'm not entirely convinced that we need anything new here.
>> The cost estimate function can detect such situations, and so can
>> the index AM at scan start --- for example, btree checks for
>> contradictory quals at scan start. There's a certain amount of
>> duplicative effort involved there perhaps, but you also have to
>> keep in mind that we don't know the values of run-time-determined
>> comparison values until scan start. So if you want certainty rather
>> than just a cost estimate, you may have to do these sorts of checks
>> at scan start.
> Ah, I didn't know about _bt_preprocess_keys(). I'm not familiar with
> this code, so please bear with me. IIUC the idea would be to add
> additional logic in gingetbitmap() / ginNewScanKey() to drop some
> quals at runtime. But that would mean that additional logic would
> also be required in BitmapHeapScan, or that all the returned bitmap
> should be artificially marked as lossy to enforce a recheck?

We have a similar solution for this problem. The idea is to avoid full index
scan inside GIN itself when we have some GIN entries, and forcibly recheck
all tuples if triconsistent() returns GIN_MAYBE for the keys that emitted no
GIN entries.

The attached patch in its current shape contain at least two ugly places:

1. We still need to initialize empty scan key to call triconsistent(), but
then we have to remove it from the list of scan keys. Simple refactoring
of ginFillScanKey() can be helpful here.

2. We need to replace GIN_SEARCH_MODE_EVERYTHING with GIN_SEARCH_MODE_ALL
if there are no GIN entries and some key requested GIN_SEARCH_MODE_ALL
because we need to skip NULLs in GIN_SEARCH_MODE_ALL. Simplest example here
is "array @> '{}'": triconsistent() returns GIN_TRUE, recheck is not forced,
and GIN_SEARCH_MODE_EVERYTHING returns NULLs that are not rechecked. Maybe
it would be better to introduce new GIN_SEARCH_MODE_EVERYTHING_NON_NULL.

Example:

CREATE TABLE test AS SELECT i::text AS t FROM generate_series(0, 999999) i;

CREATE INDEX ON test USING gin (t gin_trgm_ops);

-- master
EXPLAIN ANALYZE SELECT * FROM test WHERE LIKE '%1234%' AND t LIKE '%1%';
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on test (cost=11777.99..16421.73 rows=7999 width=32) (actual time=65.431..65.857 rows=300 loops=1)
Recheck Cond: ((t ~~ '%1234%'::text) AND (t ~~ '%1%'::text))
Rows Removed by Index Recheck: 2
Heap Blocks: exact=114
-> Bitmap Index Scan on test_t_idx (cost=0.00..11775.99 rows=7999 width=0) (actual time=65.380..65.380 rows=302 loops=1)
Index Cond: ((t ~~ '%1234%'::text) AND (t ~~ '%1%'::text))
Planning Time: 0.151 ms
Execution Time: 65.900 ms
(8 rows)

-- patched
EXPLAIN ANALYZE SELECT * FROM test WHERE t LIKE '%1234%' AND t LIKE '%1%';
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on test (cost=20.43..176.79 rows=42 width=6) (actual time=0.287..0.424 rows=300 loops=1)
Recheck Cond: ((t ~~ '%1234%'::text) AND (t ~~ '%1%'::text))
Rows Removed by Index Recheck: 2
Heap Blocks: exact=114
-> Bitmap Index Scan on test_t_idx (cost=0.00..20.42 rows=42 width=0) (actual time=0.271..0.271 rows=302 loops=1)
Index Cond: ((t ~~ '%1234%'::text) AND (t ~~ '%1%'::text))
Planning Time: 0.080 ms
Execution Time: 0.450 ms
(8 rows)

--
Nikita Glukhov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company