There is a blog below (which is for oracle), showing how the index should be chosen and it states , "Stick the columns you do range scans on last in the index, filters that get equality predicates should come first. ", and in that case we should have the PK created as in the order (transaction_id,transaction_timestamp). It's because making the range predicate as a leading column won't help use that as an access predicate but as an filter predicate thus will read more blocks and thus more IO. Does this hold true in postgres too?
I believe the analogy holds true here in postgres too and the index in this case should be on (transaction_id, transaction_timestamp).
Additionally there is another scenario in which we have the requirement to have another timestamp column (say create_timestamp) to be added as part of the primary key along with transaction_id and we are going to query this table frequently by the column create_timestamp as a range predicate. And ofcourse we will also have the range predicate filter on partition key "transaction_timestamp". But we may or may not have join/filter on column transaction_id, so in this scenario we should go for (create_timestamp,transaction_id,transaction_timestamp). because "transaction_timestamp" is set as partition key , so putting it last doesn't harm us. Will this be the correct order or any other index order is appropriate?
In this case , the index should be on ( create_timestamp,transaction_id,transaction_timestamp), considering the fact that you will always have queries with "create_timestamp" as predicate and may not have transaction_id in the query predicate.
So in the second scenario, if we keep the create_timestamp as the leading column ,is it not against the advice which the blog provides i.e. to not have the range predicate as the leading column in the index?