Update performance issues - best practice - postgresql

Question

I've just started working with PostgreSQL, I've used to work with SQL Server and I'm currently migrating some of the existing processes.
The current issue which I'm facing is the performance for an Update statement.
I'm trying to update all records from one table (e.g. MyTable_History) and set new values for some columns.
In Sql Server I've used the following syntax:
declare #NewEndDate datetime = (select dateadd(minute, -1, getdate()))
update MyTable_History
set isLastestVersion=0, ValidTo=#NewEndDate , ModifiedBy='TestSCriptSql',ModifiedTime=GETDATE()
The code which i could come up with (since I don't know how to simply use variables, therefore used a temp tbl) for PostgreSQL is:
CREATE TEMP TABLE dates AS VALUES (current_timestamp + (-1 ||' minutes')::interval);
with d as (
select th.validto as validto, th.islatestversion as islatestversion,
th.modifiedby as modifiedby, th.modifiedtime as modifiedtime, d.column1 as newvalidto
from MyTable_History th, dates d
)
update MyTable_History
set validto = d.newvalidto, islatestversion=false, modifiedby='test_update_script', modifiedtime=current_timestamp
from d
The Sql Server runs localy on my laptop (not a super config) and the PosgreSQL server runs on AWS as RDS (i don't know the exact specs).
My question is am I doing something wrong in the PostgreSql update statement? Because on a 5000+ dataset sample on Sql Server the statement is instantly performed, while on PostgreSql it takes around 50 secs to successfully finish.
Also, from my point of view it seems I've over engineered, since on Sql Server I was having 3 lines of code, while on postgreSql i'm using a CTE.
Regrards,

Answer 1

I don't see why you would need a variable to begin with. current_timestamp returns the same value throughout a transaction as documented in the manual and thus will have the same value for all updated rows.
update mytable_history
set islastestversion = 0,
validto = current_timestamp - interval '1 minute',
modifiedby = 'test_update_script',
modifiedtime = current_timestamp;
But your usage of FROM in the UPDATE statement is wrong. The semantics of using FROM in an UPDATE statement are very different between Postgres and SQL Server
The way you use it, creates a cross join between the CTE and mytable_history. (so essentially a cross join of the table with itself).
You need to have a join condition in the WHERE clause on the primary key:
with d as (...)
update MyTable_History
set validto = d.newvalidto, islatestversion=false,
modifiedby='test_update_script', modifiedtime=current_timestamp
from d
where d.pk_column = MyTable_History.pk_column;
But if you really want to simulate something like variables, you don't need the CTE:
update mytable_history
set islastestversion = 0,
validto = t.newvalidto
modifiedby = 'test_update_script',
modifiedtime = current_timestamp
from (
values (current_timestamp - interval '1 minute')
) t (newvalidto);
The above still creates a "cross join" but as the joined table (from (values ...)) only contains a single row, it's not really a cross join.