我在Postgres 9.2（数百万行）中有几个大表，我需要根据两个字段'source'（varchar）和'id'（int）的组合生成一个唯一的代码。我可以通过生成row_numbers来执行此操作：
SELECT source,id FROM tablename GROUP BY source,id
但结果可能需要一段时间才能完成。建议如果字段被索引，并且索引值的比例很小（这是我的情况），那么松散的索引扫描可能是更好的选择：http://wiki.postgresql.org/wiki/Loose_indexscan
WITH RECURSIVE
t AS (SELECT min(col) AS col FROM tablename
UNION ALL
SELECT (SELECT min(col) FROM tablename WHERE col > t.col) FROM t WHERE t.col IS NOT NULL)
SELECT col FROM t WHERE col IS NOT NULL
UNION ALL
SELECT NULL WHERE EXISTS(SELECT * FROM tablename WHERE col IS NULL);
该示例在单个字段上运行。尝试返回多个字段会产生错误：子查询必须只返回一列。一种可能性是尝试检索整个ROW - 例如SELECT ROW(min(source),min(id)...，但后来我不确定WHERE语句的语法需要看起来像处理单个行元素。
问题是：是否可以修改基于递归的代码以使用多个列，如果是，如何？我承诺使用Postgres，但看起来MySQL已经为多个列实现了松散的索引扫描：http://dev.mysql.com/doc/refman/5.1/en/group-by-optimization.html
根据建议，我正在附上我的EXPLAIN ANALYZE结果。
对于我的情况 - 我使用GROUP BY 为2列选择不同的值，它是以下内容：
HashAggregate (cost=1645408.44..1654099.65 rows=869121 width=34) (actual time=35411.889..36008.475 rows=1233080 loops=1)
-> Seq Scan on tablename (cost=0.00..1535284.96 rows=22024696 width=34) (actual time=4413.311..25450.840 rows=22025768 loops=1)
Total runtime: 36127.789 ms
(3 rows)
我不知道如何进行2列索引扫描（这是问题），但为了进行比较，在一列上使用GROUP BY，我得到：
HashAggregate (cost=1590346.70..1590347.69 rows=99 width=8) (actual time=32310.706..32310.722 rows=100 loops=1)
-> Seq Scan on tablename (cost=0.00..1535284.96 rows=22024696 width=8) (actual time=4764.609..26941.832 rows=22025768 loops=1)
Total runtime: 32350.899 ms
(3 rows)
但是对于一列上的松散索引扫描，我得到：
Result (cost=181.28..198.07 rows=101 width=8) (actual time=0.069..1.935 rows=100 loops=1)
CTE t
-> Recursive Union (cost=1.74..181.28 rows=101 width=8) (actual time=0.062..1.855 rows=101 loops=1)
-> Result (cost=1.74..1.75 rows=1 width=0) (actual time=0.061..0.061 rows=1 loops=1)
InitPlan 1 (returns $1)
-> Limit (cost=0.00..1.74 rows=1 width=8) (actual time=0.057..0.057 rows=1 loops=1)
-> Index Only Scan using tablename_id on tablename (cost=0.00..38379014.12 rows=22024696 width=8) (actual time=0.055..0.055 rows=1 loops=1)
Index Cond: (id IS NOT NULL)
Heap Fetches: 0
-> WorkTable Scan on t (cost=0.00..17.75 rows=10 width=8) (actual time=0.017..0.017 rows=1 loops=101)
Filter: (id IS NOT NULL)
Rows Removed by Filter: 0
SubPlan 3
-> Result (cost=1.75..1.76 rows=1 width=0) (actual time=0.016..0.016 rows=1 loops=100)
InitPlan 2 (returns $3)
-> Limit (cost=0.00..1.75 rows=1 width=8) (actual time=0.016..0.016 rows=1 loops=100)
-> Index Only Scan using tablename_id on tablename (cost=0.00..12811462.41 rows=7341565 width=8) (actual time=0.015..0.015 rows=1 loops=100)
Index Cond: ((id IS NOT NULL) AND (id > t.id))
Heap Fetches: 0
-> Append (cost=0.00..16.79 rows=101 width=8) (actual time=0.067..1.918 rows=100 loops=1)
-> CTE Scan on t (cost=0.00..2.02 rows=100 width=8) (actual time=0.067..1.899 rows=100 loops=1)
Filter: (id IS NOT NULL)
Rows Removed by Filter: 1
-> Result (cost=13.75..13.76 rows=1 width=0) (actual time=0.002..0.002 rows=0 loops=1)
One-Time Filter: $5
InitPlan 5 (returns $5)
-> Index Only Scan using tablename_id on tablename (cost=0.00..13.75 rows=1 width=0) (actual time=0.002..0.002 rows=0 loops=1)
Index Cond: (id IS NULL)
Heap Fetches: 0
Total runtime: 2.040 ms
完整的表定义如下所示：
CREATE TABLE tablename
(
source character(25),
id bigint NOT NULL,
time_ timestamp without time zone,
height numeric,
lon numeric,
lat numeric,
distance numeric,
status character(3),
geom geometry(PointZ,4326),
relid bigint
)
WITH (
OIDS=FALSE
);
CREATE INDEX tablename_height
ON public.tablename
USING btree
(height);
CREATE INDEX tablename_geom
ON public.tablename
USING gist
(geom);
CREATE INDEX tablename_id
ON public.tablename
USING btree
(id);
CREATE INDEX tablename_lat
ON public.tablename
USING btree
(lat);
CREATE INDEX tablename_lon
ON public.tablename
USING btree
(lon);
CREATE INDEX tablename_relid
ON public.tablename
USING btree
(relid);
CREATE INDEX tablename_sid
ON public.tablename
USING btree
(source COLLATE pg_catalog."default", id);
CREATE INDEX tablename_source
ON public.tablename
USING btree
(source COLLATE pg_catalog."default");
CREATE INDEX tablename_time
ON public.tablename
USING btree
(time_);
答案选择：
我花了一些时间来比较所提供的方法。有时这样我希望不止一个答案可以被接受，但在这种情况下，我正在给@jjanes打勾。这样做的原因是他的解决方案与最初提出的问题更紧密地匹配，并且我能够获得关于所需WHERE语句的形式的一些见解。最后，HashAggregate实际上是最快的方法（对我来说），但这是由于我的数据的性质，而不是算法的任何问题。我已经为下面的不同方法添加了EXPLAIN ANALYZE，并且将为jjanes和joop提供+1。
HashAggregate：
HashAggregate (cost=1018669.72..1029722.08 rows=1105236 width=34) (actual time=24164.735..24686.394 rows=1233080 loops=1)
-> Seq Scan on tablename (cost=0.00..908548.48 rows=22024248 width=34) (actual time=0.054..14639.931 rows=22024982 loops=1)
Total runtime: 24787.292 ms
索引扫描修改松散
CTE Scan on t (cost=13.84..15.86 rows=100 width=112) (actual time=0.916..250311.164 rows=1233080 loops=1)
Filter: (source IS NOT NULL)
Rows Removed by Filter: 1
CTE t
-> Recursive Union (cost=0.00..13.84 rows=101 width=112) (actual time=0.911..249295.872 rows=1233081 loops=1)
-> Limit (cost=0.00..0.04 rows=1 width=34) (actual time=0.910..0.911 rows=1 loops=1)
-> Index Only Scan using tablename_sid on tablename (cost=0.00..965442.32 rows=22024248 width=34) (actual time=0.908..0.908 rows=1 loops=1)
Heap Fetches: 0
-> WorkTable Scan on t (cost=0.00..1.18 rows=10 width=112) (actual time=0.201..0.201 rows=1 loops=1233081)
Filter: (source IS NOT NULL)
Rows Removed by Filter: 0
SubPlan 1
-> Limit (cost=0.00..0.05 rows=1 width=34) (actual time=0.100..0.100 rows=1 loops=1233080)
-> Index Only Scan using tablename_sid on tablename (cost=0.00..340173.38 rows=7341416 width=34) (actual time=0.100..0.100 rows=1 loops=1233080)
Index Cond: (ROW(source, id) > ROW(t.source, t.id))
Heap Fetches: 0
SubPlan 2
-> Limit (cost=0.00..0.05 rows=1 width=34) (actual time=0.099..0.099 rows=1 loops=1233080)
-> Index Only Scan using tablename_sid on tablename (cost=0.00..340173.38 rows=7341416 width=34) (actual time=0.098..0.098 rows=1 loops=1233080)
Index Cond: (ROW(source, id) > ROW(t.source, t.id))
Heap Fetches: 0
Total runtime: 250491.559 ms
合并反加入
Merge Anti Join (cost=0.00..12099015.26 rows=14682832 width=42) (actual time=48.710..541624.677 rows=1233080 loops=1)
Merge Cond: ((src.source = nx.source) AND (src.id = nx.id))
Join Filter: (nx.time_ > src.time_)
Rows Removed by Join Filter: 363464177
-> Index Only Scan using tablename_pkey on tablename src (cost=0.00..1060195.27 rows=22024248 width=42) (actual time=48.566..5064.551 rows=22024982 loops=1)
Heap Fetches: 0
-> Materialize (cost=0.00..1115255.89 rows=22024248 width=42) (actual time=0.011..40551.997 rows=363464177 loops=1)
-> Index Only Scan using tablename_pkey on tablename nx (cost=0.00..1060195.27 rows=22024248 width=42) (actual time=0.008..8258.890 rows=22024982 loops=1)
Heap Fetches: 0
Total runtime: 541750.026 ms
答案 0 :(得分：2)
相当可怕，但这似乎有效：
WITH RECURSIVE
t AS (
select a,b from (select a,b from foo order by a,b limit 1) asdf union all
select (select a from foo where (a,b) > (t.a,t.b) order by a,b limit 1),
(select b from foo where (a,b) > (t.a,t.b) order by a,b limit 1)
from t where t.a is not null)
select * from t where t.a is not null;
我真的不明白为什么需要“不为空”，因为空值首先来自何处？
答案 1 :(得分：1)
DROP SCHEMA zooi CASCADE;
CREATE SCHEMA zooi ;
SET search_path=zooi,public,pg_catalog;
CREATE TABLE tablename
( source character(25) NOT NULL
, id bigint NOT NULL
, time_ timestamp without time zone NOT NULL
, height numeric
, lon numeric
, lat numeric
, distance numeric
, status character(3)
, geom geometry(PointZ,4326)
, relid bigint
, PRIMARY KEY (source,id,time_) -- <<-- Primary key here
) WITH ( OIDS=FALSE);
-- invent some bogus data
INSERT INTO tablename(source,id,time_)
SELECT 'SRC_'|| (gs%10)::text
,gs/10
,gt
FROM generate_series(1,1000) gs
, generate_series('2013-12-01', '2013-12-07', '1hour'::interval) gt
;
为两个关键字段选择唯一值：
VACUUM ANALYZE tablename;
EXPLAIN ANALYZE
SELECT source,id,time_
FROM tablename src
WHERE NOT EXISTS (
SELECT * FROM tablename nx
WHERE nx.source =src.source
AND nx.id = src.id
AND time_ > src.time_
)
;
在此处生成此计划（Pg-9.3）：
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------
Hash Anti Join (cost=4981.00..12837.82 rows=96667 width=42) (actual time=547.218..1194.335 rows=1000 loops=1)
Hash Cond: ((src.source = nx.source) AND (src.id = nx.id))
Join Filter: (nx.time_ > src.time_)
Rows Removed by Join Filter: 145000
-> Seq Scan on tablename src (cost=0.00..2806.00 rows=145000 width=42) (actual time=0.010..210.810 rows=145000 loops=1)
-> Hash (cost=2806.00..2806.00 rows=145000 width=42) (actual time=546.497..546.497 rows=145000 loops=1)
Buckets: 16384 Batches: 1 Memory Usage: 9063kB
-> Seq Scan on tablename nx (cost=0.00..2806.00 rows=145000 width=42) (actual time=0.006..259.864 rows=145000 loops=1)
Total runtime: 1197.374 ms
(9 rows)
一旦数据超出work_mem，哈希联接可能会消失：
Merge Anti Join (cost=0.83..8779.56 rows=29832 width=120) (actual time=0.981..2508.912 rows=1000 loops=1)
Merge Cond: ((src.source = nx.source) AND (src.id = nx.id))
Join Filter: (nx.time_ > src.time_)
Rows Removed by Join Filter: 184051
-> Index Scan using tablename_sid on tablename src (cost=0.41..4061.57 rows=32544 width=120) (actual time=0.055..250.621 rows=145000 loops=1)
-> Index Scan using tablename_sid on tablename nx (cost=0.41..4061.57 rows=32544 width=120) (actual time=0.008..603.403 rows=328906 loops=1)
Total runtime: 2510.505 ms