我有一个查询，并且专门为此查询创建了索引。但是我发现，如果使用某些特定值，查询将停止快速运行，并进行全面扫描。
这是快速执行的情况：
explain analyze SELECT
v.valtr_id,
v.block_num,
v.from_id,
v.to_id,
v.from_balance::text,
v.to_balance::text
FROM value_transfer v
WHERE
(v.block_num<=2748053) AND
(
(v.to_id=639291) OR
(v.from_id=639291)
)
ORDER BY
v.block_num DESC,v.valtr_id DESC
LIMIT 1
Limit (cost=23054.03..23054.03 rows=1 width=30) (actual time=1.464..1.465 rows=1 loops=1)
-> Sort (cost=23054.03..23068.94 rows=5964 width=30) (actual time=1.462..1.462 rows=1 loops=1)
Sort Key: block_num DESC, valtr_id DESC
Sort Method: top-N heapsort Memory: 25kB
-> Bitmap Heap Scan on value_transfer v (cost=144.85..23024.21 rows=5964 width=30) (actual time=1.397..1.437 rows=3 loops=1)
Recheck Cond: ((to_id = 639291) OR (from_id = 639291))
Filter: (block_num <= 2748053)
Heap Blocks: exact=3
-> BitmapOr (cost=144.85..144.85 rows=5964 width=0) (actual time=1.339..1.339 rows=0 loops=1)
-> Bitmap Index Scan on vt_to_id_idx (cost=0.00..40.42 rows=1580 width=0) (actual time=0.755..0.755 rows=1 loops=1)
Index Cond: (to_id = 639291)
-> Bitmap Index Scan on vt_from_id_idx (cost=0.00..101.45 rows=4384 width=0) (actual time=0.580..0.580 rows=2 loops=1)
Index Cond: (from_id = 639291)
Planning time: 0.499 ms
Execution time: 1.556 ms
(15 rows)
但是，如果我将值199658作为查询的输入，它将使用不同的搜索算法：
explain analyze SELECT
v.valtr_id,
v.block_num,
v.from_id,
v.to_id,
v.from_balance::text,
v.to_balance::text
FROM value_transfer v
WHERE
(v.block_num<=2748053) AND
(
(v.to_id=199658) OR
(v.from_id=199658)
)
ORDER BY
v.block_num DESC,v.valtr_id DESC
LIMIT 1 ;
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.57..6462.99 rows=1 width=30) (actual time=614109.855..614109.856 rows=1 loops=1)
-> Index Scan Backward using bnum_valtr_idx on value_transfer v (cost=0.57..200845479.66 rows=31079 width=30) (actual time=614109.853..614109.853 rows=1 loops=1)
Index Cond: (block_num <= 2748053)
Filter: ((to_id = 199658) OR (from_id = 199658))
Rows Removed by Filter: 101190609
Planning time: 0.515 ms
Execution time: 614109.920 ms
(7 rows)
为什么会这样？我认为，一旦为查询创建了索引，执行将始终采用相同的路径，但事实并非如此。如何确保Postgres在每次搜索中始终使用相同的算法？
我什至以为会发生这种情况，因为可能不是很干净地建立了索引并重建了主索引：
postgres=> drop index bnum_valtr_idx;
DROP INDEX
postgres=> CREATE INDEX bnum_valtr_idx ON public.value_transfer USING btree (block_num DESC, valtr_id DESC);
CREATE INDEX
postgres=>
但是，这没有任何改变。
我的表定义是：
CREATE TABLE value_transfer (
valtr_id BIGSERIAL PRIMARY KEY,
tx_id BIGINT REFERENCES transaction(tx_id) ON DELETE CASCADE ON UPDATE CASCADE,
block_id INT REFERENCES block(block_id) ON DELETE CASCADE ON UPDATE CASCADE,
block_num INT NOT NULL,
from_id INT NOT NULL,
to_id INT NOT NULL,
value NUMERIC DEFAULT 0,
from_balance NUMERIC DEFAULT 0,
to_balance NUMERIC DEFAULT 0,
kind CHAR NOT NULL,
depth INT DEFAULT 0,
error TEXT NOT NULL
);
postgres=> SELECT * FROM pg_indexes WHERE tablename = 'value_transfer';
schemaname | tablename | indexname | tablespace | indexdef
------------+----------------+---------------------+------------+--------------------------------------------------------------------------------------------------
public | value_transfer | bnum_valtr_idx | | CREATE INDEX bnum_valtr_idx ON public.value_transfer USING btree (block_num DESC, valtr_id DESC)
public | value_transfer | value_transfer_pkey | | CREATE UNIQUE INDEX value_transfer_pkey ON public.value_transfer USING btree (valtr_id)
public | value_transfer | vt_tx_from_idx | | CREATE INDEX vt_tx_from_idx ON public.value_transfer USING btree (tx_id)
public | value_transfer | vt_block_num_idx | | CREATE INDEX vt_block_num_idx ON public.value_transfer USING btree (block_num)
public | value_transfer | vt_from_id_idx | | CREATE INDEX vt_from_id_idx ON public.value_transfer USING btree (from_id)
public | value_transfer | vt_to_id_idx | | CREATE INDEX vt_to_id_idx ON public.value_transfer USING btree (to_id)
public | value_transfer | vt_block_id_idx | | CREATE INDEX vt_block_id_idx ON public.value_transfer USING btree (block_id)
(7 rows)
postgres=>
答案 0 :(得分：0)
可能是一个值在一个栏中，反之亦然。无论如何，在不同列上使用OR会导致性能问题而臭名昭著，因为查询计划只能使用一个索引，但是OR要求使用两个索引来检查两列都很快，因此将使用其索引检查一列，但另一列需要进行扫描。
解决此问题的方法是将查询分解为一个联合。
尝试一下：
SELECT * FROM (
SELECT
valtr_id,
block_num,
from_id,
to_id,
from_balance::text,
to_balance::text
FROM value_transfer
WHERE block_num<=2748053
AND to_id=199658
UNION ALL
SELECT
valtr_id,
block_num,
from_id,
to_id,
from_balance::text,
to_balance::text
FROM value_transfer
WHERE block_num<=2748053
AND from_id=199658
) x
ORDER BY block_num DESC, valtr_id DESC
LIMIT 1