On Sat, Jun 8, 2024 at 7:03 PM veem v <veema0000@xxxxxxxxx> wrote:
Hi ,It's postgres version 15.4. A table is daily range partitioned on a column transaction_timestamp. It has a unique identifier which is the ideal for primary key (say transaction_id) , however as there is a limitation in which we have to include the partition key as part of the primary key, so it has to be a composite index. Either it has to be (transaction_id,transaction_timestamp) or ( transaction_timestamp, transaction_id). But which one should we go for, if both of the columns get used in all the queries?
We will always be using transaction_timestamp as mostly a range predicate filter/join in the query and the transaction_id will be mostly used as a join condition/direct filter in the queries. So we were wondering, which column should we be using as a leading column in this index?
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?
https://ctandrewsayer.wordpress.com/2017/03/24/the-golden-rule-of-indexing/
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.
