为啥我在 postgresql 中的视图不使用索引？
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【中文标题】为啥我在 postgresql 中的视图不使用索引？【英文标题】：why does my view in postgresql not use the index?为什么我在 postgresql 中的视图不使用索引？ 【发布时间】：2013-08-08 19:48:05 【问题描述】：我有一个大表（星形目录），其中有一个子集。我实现了子集 作为两个表的联合，我使用交叉索引。
问题是视图中的查询似乎没有使用索引，时间与扫描表的时间相同。
对大表的查询很快：
select count(*) from ucac4 where rnm in (select ucac4_rnm from grid_catalog limit 5);
count
-------
5
(1 row)
Time: 12.132 ms
针对视图的查询不会很快进行，尽管我希望它会。
select count(*) from grid_catalog_view where ident in (select ucac4_rnm from grid_catalog limit 5);
count
-------
5
(1 row)
Time: 1056237.045 ms
对这个查询的解释：
Aggregate (cost=23175810.51..23175810.52 rows=1 width=0)
-> Hash Join (cost=23081888.41..23172893.67 rows=1166734 width=0)
Hash Cond: (ucac4.rnm = public.grid_catalog.ucac4_rnm)
-> Unique (cost=23081888.17..23140224.87 rows=2333468 width=44)
-> Sort (cost=23081888.17..23087721.84 rows=2333468 width=44)
Sort Key: ucac4.ra, ucac4."dec", ucac4.pmrac, ucac4.pmdc, ucac4.rnm, ucac4.nest4, ucac4.nest6, ucac4.nest7, public.grid_catalog.subset
-> Append (cost=63349.87..22763295.24 rows=2333468 width=44)
-> Hash Join (cost=63349.87..22738772.75 rows=2333467 width=44)
Hash Cond: (ucac4.rnm = public.grid_catalog.ucac4_rnm)
-> Seq Scan on ucac4 (cost=0.00..16394129.04 rows=455124304 width=40)
-> Hash (cost=34048.69..34048.69 rows=2344094 width=8)
-> Seq Scan on grid_catalog (cost=0.00..34048.69 rows=2344094 width=8)
Filter: (petrov_prikey IS NULL)
-> Hash Join (cost=415.51..1187.80 rows=1 width=36)
Hash Cond: (petrov.prikey = public.grid_catalog.petrov_prikey)
-> Seq Scan on petrov (cost=0.00..709.15 rows=7215 width=32)
-> Hash (cost=282.08..282.08 rows=10675 width=8)
-> Index Scan using grid_catalog_petrov_prikey_idx on grid_catalog (cost=0.00..282.08 row
s=10675 width=8)
-> Hash (cost=0.18..0.18 rows=5 width=4)
-> HashAggregate (cost=0.13..0.18 rows=5 width=4)
-> Limit (cost=0.00..0.07 rows=5 width=4)
-> Seq Scan on grid_catalog (cost=0.00..34048.69 rows=2354769 width=4)
(22 rows)
解释分析（评论中的请求）是：
Aggregate (cost=23175810.51..23175810.52 rows=1 width=0) (actual time=1625067.627..1625067.628 rows=1 loops=1)
-> Hash Join (cost=23081888.41..23172893.67 rows=1166734 width=0) (actual time=1621395.200..1625067.618 rows=5 loops=1)
Hash Cond: (ucac4.rnm = public.grid_catalog.ucac4_rnm)
-> Unique (cost=23081888.17..23140224.87 rows=2333468 width=44) (actual time=1620897.932..1624102.849 rows=1597359 loops
=1)
-> Sort (cost=23081888.17..23087721.84 rows=2333468 width=44) (actual time=1620897.928..1622191.358 rows=1597359 l
oops=1)
Sort Key: ucac4.ra, ucac4."dec", ucac4.pmrac, ucac4.pmdc, ucac4.rnm, ucac4.nest4, ucac4.nest6, ucac4.nest7, pu
blic.grid_catalog.subset
Sort Method: external merge Disk: 87536kB
-> Append (cost=63349.87..22763295.24 rows=2333468 width=44) (actual time=890293.619..1613769.160 rows=15973
59 loops=1)
-> Hash Join (cost=63349.87..22738772.75 rows=2333467 width=44) (actual time=890293.617..1611550.313 r
ows=1590144 loops=1)
Hash Cond: (ucac4.rnm = public.grid_catalog.ucac4_rnm)
-> Seq Scan on ucac4 (cost=0.00..16394129.04 rows=455124304 width=40) (actual time=886086.630..1
359934.589 rows=113780093 loops=1)
-> Hash (cost=34048.69..34048.69 rows=2344094 width=8) (actual time=4203.785..4203.785 rows=1590
144 loops=1)
-> Seq Scan on grid_catalog (cost=0.00..34048.69 rows=2344094 width=8) (actual time=0.014.
.2813.031 rows=1590144 loops=1)
Filter: (petrov_prikey IS NULL)
-> Hash Join (cost=415.51..1187.80 rows=1 width=36) (actual time=101.604..165.749 rows=7215 loops=1)
Hash Cond: (petrov.prikey = public.grid_catalog.petrov_prikey)
-> Seq Scan on petrov (cost=0.00..709.15 rows=7215 width=32) (actual time=58.280..108.043 rows=7
215 loops=1)
-> Hash (cost=282.08..282.08 rows=10675 width=8) (actual time=43.276..43.276 rows=7215 loops=1)
-> Index Scan using grid_catalog_petrov_prikey_idx on grid_catalog (cost=0.00..282.08 rows
=10675 width=8) (actual time=19.387..37.533 rows=7215 loops=1)
-> Hash (cost=0.18..0.18 rows=5 width=4) (actual time=0.035..0.035 rows=5 loops=1)
-> HashAggregate (cost=0.13..0.18 rows=5 width=4) (actual time=0.026..0.030 rows=5 loops=1)
-> Limit (cost=0.00..0.07 rows=5 width=4) (actual time=0.009..0.017 rows=5 loops=1)
-> Seq Scan on grid_catalog (cost=0.00..34048.69 rows=2354769 width=4) (actual time=0.007..0.009 rows=
5 loops=1)
Total runtime: 1625108.504 ms
(24 rows)
Time: 1625466.830 ms
要查看浏览视图的时间：
select count(*) from grid_catalog_view;
count
---------
1597359
(1 row)
Time: 1033732.786 ms
我的观点定义为：
PS1=# \d grid_catalog_view
View "public.grid_catalog_view"
Column | Type | Modifiers
--------+------------------+-----------
ra | double precision |
dec | double precision |
pmrac | integer |
pmdc | integer |
ident | integer |
nest4 | integer |
nest6 | integer |
nest7 | integer |
subset | integer |
View definition:
SELECT ucac4.ra, ucac4."dec", ucac4.pmrac, ucac4.pmdc, ucac4.rnm AS ident, ucac4.nest4, ucac4.nest6, ucac4.nest7, grid_catalog.subset
FROM ucac4, grid_catalog
WHERE ucac4.rnm = grid_catalog.ucac4_rnm AND grid_catalog.petrov_prikey IS NULL
UNION
SELECT petrov.ra, petrov."dec", 0 AS pmrac, 0 AS pmdc, grid_catalog.petrov_prikey AS ident, petrov.nest4, petrov.nest6, petrov.nest7, grid_catalog.subset
FROM petrov, grid_catalog
WHERE petrov.prikey = grid_catalog.petrov_prikey AND grid_catalog.ucac4_rnm IS NULL;
大表定义为：
PS1=# \d ucac4
Table "public.ucac4"
Column | Type | Modifiers
----------+------------------+-----------
radi | bigint |
spdi | bigint |
magm | smallint |
maga | smallint |
sigmag | smallint |
objt | smallint |
cdf | smallint |
... deleted entries not of relavance ...
ra | double precision |
dec | double precision |
x | double precision |
y | double precision |
z | double precision |
nest4 | integer |
nest6 | integer |
nest7 | integer |
Indexes:
"ucac4_pkey" PRIMARY KEY, btree (rnm)
"q3c_ucac4_idx" btree (q3c_ang2ipix(ra, "dec")) CLUSTER
"ucac4_nest4_idx" btree (nest4)
"ucac4_nest6_idx" btree (nest6)
"ucac4_nest7_idx" btree (nest7)
Referenced by:
TABLE "grid_catalog" CONSTRAINT "grid_catalog_ucac4_rnm_fkey" FOREIGN KEY (ucac4_rnm) REFERENCES ucac4(rnm)
知道为什么我的索引似乎没有被使用吗？
【问题讨论】：
什么索引？我没有看到任何索引定义。 我忘记包含表模式。固定。explain analyze 更可取，因此可以看到任何行数错误估计等。
【参考方案1】：
据我所知，这是 postgres 的一个限制 - 很难避免以这种方式扫描联合上的整个表。
见：
https://www.postgresql-archive.org/Poor-plan-when-joining-against-a-union-containing-a-join-td5747690.html
和
https://www.postgresql-archive.org/Pushing-IN-subquery-down-through-UNION-ALL-td3398684.html
也可能相关
https://dba.stackexchange.com/questions/47572/in-postgresql-9-3-union-view-with-where-clause-not-taken-into-account
基本上 - 我想你需要重新审视你的视图定义！很抱歉没有明确的解决方案。
【讨论】：
前 2 个链接已损坏 修复了链接 - 该站点已将通用 nabble 域名替换为自定义域名，如更新的链接中所示。以上是关于为啥我在 postgresql 中的视图不使用索引？的主要内容，如果未能解决你的问题，请参考以下文章
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