slow entity framework query , but fast Generated SQL - sql

Question

please consider this model
it's for a fitness center management app
ADHERANT is the members table
INSCRIPTION is the subscription table
SEANCE is the individual sessions table
the seance table contain very fews rows (around 7000)
now the query :
var q = from n in ctx.SEANCES
select new SeanceJournalType()
{
ID_ADHERANT = n.INSCRIPTION.INS_ID_ADHERANT,
ADH_NOM = n.INSCRIPTION.ADHERANT.ADH_NOM,
ADH_PRENOM = n.INSCRIPTION.ADHERANT.ADH_PRENOM,
ADH_PHOTO = n.INSCRIPTION.ADHERANT.ADH_PHOTO,
SEA_DEBUT = n.SEA_DEBUT
};
var h = q.ToList();
this take around 3 seconds wich is an eternity,
the same generated SQL query is almost instantaneous
SELECT
1 AS "C1",
"C"."INS_ID_ADHERANT" AS "INS_ID_ADHERANT",
"E"."ADH_NOM" AS "ADH_NOM",
"E"."ADH_PRENOM" AS "ADH_PRENOM",
"E"."ADH_PHOTO" AS "ADH_PHOTO",
"B"."SEA_DEBUT" AS "SEA_DEBUT"
FROM "TMP_SEANCES" AS "B"
LEFT OUTER JOIN "INSCRIPTIONS" AS "C" ON "B"."INS_ID_INSCRIPTION" = "C"."ID_INSCRIPTION"
LEFT OUTER JOIN "ADHERANTS" AS "E" ON "C"."INS_ID_ADHERANT" = "E"."ID_ADHERANT"
any idea on what's going on please, or how to fix that ?
thanks

Answer 1

it needs some research to optimize this :
if you neglect the data transfer from the db to the server then
as Ivan Stoev Suggested calling the ToList method is the expensive part
as for improving the performance it depends on your needs:
1.if you need add-delete functionality at the server side it is probably best to stick with the list
2.if no need for add-delete then consider ICollection ,, or even better
3.if you have more conditions which will customize the query even more best use IQuerable
customizing the query like selecting a single record based on a condition :
var q = from n in ctx.SEA.... // your query without ToList()
q.where(x=>"some condition") //let`s say x.Id=1
only one record will be transferred from the database to the server
but with the ToList Conversion all the records will be transferred to the server then the condition will be calculated
although it is not always the best to use IQuerable it depends on your business need
for more references check this and this