我想创建一个地图样式的索引，例如Golang中的map或Javascript中的关联数组。我需要映射的键为account_id，映射的value为记录的有序列表。可能吗？我发现Postgres具有expression索引，但是我不知道如何从具有OR条件的表达式中组装映射。
我真实的例子：
我有一个包含帐户转账的表，当前正在使用此查询来获取帐户的最新余额：
SELECT
valtr_id,
from_id,
to_id,
from_balance,
to_balance
FROM value_transfer v
WHERE
(v.block_num<=2435013) AND
(
(v.to_id = 22479) OR
(v.from_id = 22479)
)
ORDER BY v.block_num DESC,v.valtr_id DESC LIMIT 1
必须使用OR，因为帐户可能有传出转账（设置了from_id或传入转账（设置了to_id）。如果我有一个关联数组索引，它将保存account_id（将作为条件导出：if from_id==account_id OR to_id=account_id），则Postgres可以使用account_id查找此索引以获取记录列表，这将已经排序。由于索引已经考虑了OR条件，因此我不需要使用from_id=22479来构建记录列表，然后使用to_id=22479进行比较，以比较哪个记录具有最新的时间戳以获取最新的余额。的帐户，就像我现在对当前查询所做的一样。 （block_num是进行转移的区块链区块）
当前，此查询需要花费大量时间来建立具有1亿条记录的庞大数据库，下面是它的EXPLAIN ANALYZE：
postgres-> \g
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=1592973.24..1592973.24 rows=1 width=31) (actual time=86448.709..86448.710 rows=1 loops=1)
-> Sort (cost=1592973.24..1595439.02 rows=986312 width=31) (actual time=86448.707..86448.707 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=35340.86..1588041.68 rows=986312 width=31) (actual time=851.598..85082.223 rows=1387411 loops=1)
Recheck Cond: ((to_id = 22479) OR (from_id = 22479))
Filter: (block_num <= 2435013)
Rows Removed by Filter: 298923
Heap Blocks: exact=274549
-> BitmapOr (cost=35340.86..35340.86 rows=1291543 width=0) (actual time=729.917..729.917 rows=0 loops=1)
-> Bitmap Index Scan on vt_to_id_idx (cost=0.00..27233.03 rows=1004862 width=0) (actual time=575.558..575.558 rows=1364039 loops=1)
Index Cond: (to_id = 22479)
-> Bitmap Index Scan on vt_from_id_idx (cost=0.00..7614.68 rows=286681 width=0) (actual time=154.356..154.356 rows=352366 loops=1)
Index Cond: (from_id = 22479)
Planning time: 0.367 ms
Execution time: 86448.817 ms
(16 rows)
postgres=>
该表的定义如下：
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
);
CREATE INDEX vt_tx_idx ON value_transfer USING btree ("tx_id");
CREATE INDEX vt_block_num_idx ON value_transfer USING btree ("block_num");
CREATE INDEX vt_block_id_idx ON value_transfer USING btree ("block_id");
CREATE INDEX vt_from_id_idx ON value_transfer USING btree ("from_id");
CREATE INDEX vt_to_id_idx ON value_transfer USING btree ("to_id");
from_id和to_id是帐户表的外键：
CREATE TABLE account (
account_id SERIAL PRIMARY KEY,
owner_id INT NOT NULL DEFAULT 0,
last_balance NUMERIC DEFAULT 0,
num_tx BIGINT DEFAULT 0,
ts_created INT DEFAULT 0,
block_created INT DEFAULT 0,
deleted SMALLINT DEFAULT 0,
block_sd INT DEFAULT 0,
address TEXT NOT NULL UNIQUE
);
编辑：
Lukasz提出的UNION查询与旧查询的执行计划比较
UNION查询：
Limit (cost=1668089.09..1668089.10 rows=1 width=32) (actual time=6115.484..6115.485 rows=1 loops=1)
-> Sort (cost=1668089.09..1671668.88 rows=1431916 width=32) (actual time=6115.483..6115.483 rows=1 loops=1)
Sort Key: v.block_num DESC, v.valtr_id DESC
Sort Method: top-N heapsort Memory: 25kB
-> Append (cost=21229.61..1660929.51 rows=1431916 width=32) (actual time=255.166..5446.818 rows=1413507 loops=1)
-> Bitmap Heap Scan on value_transfer v (cost=21229.61..1229731.99 rows=1134056 width=32) (actual time=255.165..4312.769 rows=1102867 loops=1)
Recheck Cond: (to_id = 22479)
Rows Removed by Index Recheck: 9412580
Filter: (block_num <= 2435013)
Heap Blocks: exact=32392 lossy=132879
-> Bitmap Index Scan on vt_to_id_idx (cost=0.00..20946.10 rows=1134071 width=0) (actual time=241.632..241.632 rows=1102867 loops=1)
Index Cond: (to_id = 22479)
-> Index Scan using vt_from_id_idx on value_transfer v_1 (cost=0.57..416878.36 rows=297860 width=32) (actual time=0.056..952.883 rows=310640 loops=1)
Index Cond: (from_id = 22479)
Filter: (block_num <= 2435013)
Planning time: 0.319 ms
Execution time: 6115.539 ms
(17 rows)
OR OR CONDITION查询（我的原始查询）：
Limit (cost=1276124.75..1276124.75 rows=1 width=32) (actual time=7860.439..7860.440 rows=1 loops=1)
-> Sort (cost=1276124.75..1279694.24 rows=1427797 width=32) (actual time=7860.437..7860.437 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=27162.56..1268985.76 rows=1427797 width=32) (actual time=304.197..7194.825 rows=1387411 loops=1)
Recheck Cond: ((to_id = 22479) OR (from_id = 22479))
Rows Removed by Index Recheck: 13260750
Filter: (block_num <= 2435013)
Heap Blocks: exact=37782 lossy=186738
-> BitmapOr (cost=27162.56..27162.56 rows=1431937 width=0) (actual time=288.359..288.359 rows=0 loops=1)
-> Bitmap Index Scan on vt_to_id_idx (cost=0.00..20946.11 rows=1134072 width=0) (actual time=216.708..216.708 rows=1102867 loops=1)
Index Cond: (to_id = 22479)
-> Bitmap Index Scan on vt_from_id_idx (cost=0.00..5502.55 rows=297865 width=0) (actual time=71.649..71.649 rows=310640 loops=1)
Index Cond: (from_id = 22479)
Planning time: 0.257 ms
Execution time: 7860.481 ms
(16 rows)
使用UNION查询，执行速度提高了1.7秒。
编辑2
这个简单的查询非常快。
EXPLAIN ANALYZE
SELECT
valtr_id,
from_id,
to_id,
from_balance,
to_balance,
block_num
FROM value_transfer v
WHERE v.block_num<=2435013 AND v.from_id = 22479
LIMIT 1
Limit (cost=0.57..1.97 rows=1 width=32) (actual time=0.047..0.047 rows=1 loops=1)
-> Index Scan using vt_from_id_idx on value_transfer v (cost=0.57..416878.36 rows=297860 width=32) (actual time=0.045..0.045 rows=1 loops=1)
Index Cond: (from_id = 22479)
Filter: (block_num <= 2435013)
Planning time: 0.392 ms
Execution time: 0.089 ms
(6 rows)
但如果是OR版，则需要花费更多时间。告诉Postgres在两个查询之间建立UNION一定有问题。也许编写PL / PGSQL会更好
答案 0 :(得分：1)
我会尝试将其重写为：
--or-expansion
SELECT
valtr_id,
from_id,
to_id,
from_balance,
to_balance,
block_num
FROM value_transfer v
WHERE v.block_num<=2435013 AND v.to_id = 22479
UNION ALL
SELECT
valtr_id,
from_id,
to_id,
from_balance,
to_balance,
block_num
FROM value_transfer v
WHERE v.block_num<=2435013 AND v.from_id = 22479
ORDER BY block_num DESC,valtr_id DESC LIMIT 1
并添加两个索引：
CREATE INDEX idx_1 ON value_transfer(from_id, block_num DESC);
CREATE INDEX idx_2 ON value_transfer(to_id, block_num DESC);
答案 1 :(得分：0)
请尝试一下，它一定要好得多。
(
SELECT valtr_id,
from_id,
to_id,
from_balance,
to_balance,
block_num
FROM value_transfer v
WHERE v.block_num<=2435013 AND
v.to_id = 22479
ORDER BY block_num DESC,valtr_id DESC
LIMIT 1
)
UNION ALL
(
SELECT valtr_id,
from_id,
to_id,
from_balance,
to_balance,
block_num
FROM value_transfer v
WHERE v.block_num<=2435013 AND
v.from_id = 22479
ORDER BY block_num DESC,valtr_id DESC
LIMIT 1
)
ORDER BY block_num DESC,valtr_id DESC
LIMIT 1