dlaczego mój widok w postgresql nie korzysta z indeksu?

Question

Mam dużą tabelę (katalog gwiazd), z której mam podzestaw. Zaimplementowałem podzbiór jako związek dwóch tabel, w których korzystam z indeksu krzyżowego.

Problem polega na tym, że zapytanie z widoku nie korzysta z indeksu, czas zajmuje to samo, co skanowanie w tabeli.

Zapytanie przeciwko dużym stole idzie szybko:

select count(*) from ucac4 where rnm in (select ucac4_rnm from grid_catalog limit 5); 
count 
------- 
    5 
(1 row) 

Time: 12.132 ms 

Zapytanie przeciwko widoku nie idzie szybko, chociaż chciałbym oczekiwać, że.

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 

wyjaśniania tego zapytania yeilds:

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) 

wyjaśniania analizować (wniosek w komentarzu) wynosi:

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 

Aby zobaczyć czas skanowania w widoku:

select count(*) from grid_catalog_view; 
    count 
--------- 
1597359 
(1 row) 

Time: 1033732.786 ms 

Mój widok to d efined jak:

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; 

Duży stół jest zdefiniowany jako:

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) 

Każdy pomysł, dlaczego mój indeks nie wydają się być używane?

Answer 1

O ile widzę, jest to ograniczenie w postgresie - trudno jest uniknąć skanowania całego stołu na związku w ten sposób.

Patrz:

http://postgresql.1045698.n5.nabble.com/Poor-plan-when-joining-against-a-union-containing-a-join-td5747690.html

i

http://postgresql.1045698.n5.nabble.com/Pushing-IN-subquery-down-through-UNION-ALL-td3398684.html

a także być może związane

https://dba.stackexchange.com/questions/47572/in-postgresql-9-3-union-view-with-where-clause-not-taken-into-account

Zasadniczo - Chyba trzeba aby ponownie przejrzeć definicję widoku! Przepraszamy za brak ostatecznego rozwiązania.