为什么LinqToEntities跳过/采取oracle实现这么慢

Question

Context is an Oracle Database, Entity Framework 5, LinqToEntities, Database First.

I'm trying to implement pagination over some large tables, my linqToEntities query looks like this :

context.MyDbSet
    .Include("T2")
    .Include("T3")
    .Include("T4")
    .Include("T5")
    .Include("T6")
    .Where(o => o.T3 != null)
    .OrderBy(o => o.Id)
    .Skip(16300)
    .Take(50);

Fact is, depending on how many records i wanna skip (0 or 16300) it goes from 0.08s to 10.7s.

It seems weird to me so i checked the generated SQL and here is how it looks like :

SELECT * 
FROM ( 
SELECT 
[...]
FROM ( SELECT [...] row_number() OVER (ORDER BY "Extent1"."Id" ASC) AS "row_number"
    FROM      T1 "Extent1"
    LEFT OUTER JOIN T2 "Extent2" ON [...]
    LEFT OUTER JOIN T3 "Extent3" ON [...]
    LEFT OUTER JOIN T4 "Extent4" ON [...]
    LEFT OUTER JOIN T5 "Extent5" ON [...]
    LEFT OUTER JOIN T6 "Extent6" ON [...]
    WHERE ("Extent1"."SomeId" IS NOT NULL)
)  "Filter1"
WHERE ("Filter1"."row_number" > 16300)
ORDER BY "Filter1"."Id" ASC
)
WHERE (ROWNUM <= (50) )

I checked if it actually was Oracle who took time through SQL Developper, and it is.

I then checked the execution plan and it's what appears :

All i actually understand is that there's no STOPKEY for the first filter over row_number and that it probably fetch the whole sub query.

I don't really know where to look at, if i'm not mistaken, the request is generated by ODT/ODP/.. and thus, should be optimized for an Oracle DB.. (and i can't change it myself)

Maybe my database model is rotten and i could add whetever indexes or optimization for it to work better ?

Answer 1

You see all those UNIQUE SCAN subqueries? That's doing a UNIQUE on every single one of those tables.

You should have a one-to-many relationship with T1 being the parent, and any tables you need to be in the query to have a FOREIGN KEY relationship to an indexed ID primary key column on T1.

Then you can use Join instead of Include and the subqueries will be unnecessary.