primary key is loaded on Clickhouse start

primary_key_and_memory_allocation.sql

create table idx( A Int64, D Date) Engine=MergeTree order by A partition by D;

insert into idx select number, today() from numbers(1e8);
insert into idx select number, today()-1 from numbers(1e8);

select sum(primary_key_bytes_in_memory), sum(primary_key_bytes_in_memory_allocated) 
from system.parts where table = 'idx' and active;
┌─sum(primary_key_bytes_in_memory)─┬─sum(primary_key_bytes_in_memory_allocated)─┐
│                           195360 │                                     264192 │
└──────────────────────────────────┴────────────────────────────────────────────┘

detach table idx;
attach table idx;

select sum(primary_key_bytes_in_memory), sum(primary_key_bytes_in_memory_allocated) 
from system.parts where table = 'idx' and active;
┌─sum(primary_key_bytes_in_memory)─┬─sum(primary_key_bytes_in_memory_allocated)─┐
│                           195360 │                                     264192 │
└──────────────────────────────────┴────────────────────────────────────────────┘

systemctl restart clickhouse-server


select sum(primary_key_bytes_in_memory), sum(primary_key_bytes_in_memory_allocated) 
from system.parts where table = 'idx' and active;
┌─sum(primary_key_bytes_in_memory)─┬─sum(primary_key_bytes_in_memory_allocated)─┐
│                           195360 │                                     264192 │
└──────────────────────────────────┴────────────────────────────────────────────┘