How to decorate Expression?

Question

.NET

Asp.Net MVC, MS Sql, Entity Framework
There is a query that aggregates a dozen large tables. It looks something like this:

_db.Table.Where(t=>...)
  .Select(t=>new Model{
    Field1 = t.SomeTable.Field
    Field2 = t.SomeTable2.Field
    ...итд, около 30ти полей...}).ToList();

It all works not very fast, the load on the database creates a noticeable load.
The question is this:
It often happens that some fields are not needed, and, accordingly, it is not necessary to aggregate some tables, and therefore it is necessary to make a "basic" Expression and decorate it with the necessary additional fields.
It is theoretically possible to simply describe all possible options, but the variability is large.
Is there any way to decorate an Expression? Or any other ideas how to solve this problem?

Answer 1

Problems like this come up on a regular basis. I build the text of the SQL query manually, without any Linq to SQL.
Depending on the current needs, I include in the text of the SQL query those tables, conditions, groupings, etc. that are needed.

Answer 2

need to create stored procedures