efcore.pg | Entity Framework Core provider for PostgreSQL | SQL Database library

The usual way to use the Trigram plugin is to take a reference on the Npgsql.EntityFrameworkCore.PostgreSQL.Trigrams nuget package, and call UseTrigrams in your OnModelConfiguring:

First, you may want to look at the other method, i.e. setting up a TsVector column with HasGeneratedTsVectorColumn.

Regardless, p.Title + " " + p.Description definitely isn't evaluated before being translated to SQL - that can't happen assuming p refers to a database column. If you turn on SQL logging, you should see the exact SQL being generated by EF Core against your database. To be extra sure that the query uses your expression index, you can use EXPLAIN on that SQL and examine the query plan.

Linha's Id is automatically discovered by EF Core and set up as an auto-incrementing key - that means that database is responsible for assigning ID values. However, in your seeding you are assigning the IDs yourself via a constructor (1, 2, 3). PostgreSQL does not update the auto-increment value, so the next Linha you try to insert will get an auto-generated key of 1, which conflicts with the seeded value.

To make this work, remove the id constructor of Linha and leave it uninitialized. This will make PostgreSQL generate auto-increment values for your seeded values as well, and all should work.