For example, I have table users(id, username, email).
Is there any difference between following queries ? (maybe performance, or anything else)
SELECT id, username, email FROM users
and
SELECT id, email, username FROM users
Generally, does column order matter creating PostgreSQL tables ?
The order of columns doesn't matter in creating tables in PostgreSQL, but it does matter sometimes in creating indexes in PostgreSQL.
PostgreSQL implements primary keys with an underlying unique index. If the primary key contains more than one column, then PostgreSQL creates the underlying index with columns in the order you declare them. Some queries against part of that primary key will be able to use the index; some will not.
This quote is from the docs on multicolumn indexes.
A multicolumn B-tree index can be used with query conditions that involve any subset of the index's columns, but the index is most efficient when there are constraints on the leading (leftmost) columns. The exact rule is that equality constraints on leading columns, plus any inequality constraints on the first column that does not have an equality constraint, will be used to limit the portion of the index that is scanned. Constraints on columns to the right of these columns are checked in the index, so they save visits to the table proper, but they do not reduce the portion of the index that has to be scanned. For example, given an index on (a, b, c) and a query condition WHERE a = 5 AND b >= 42 AND c < 77, the index would have to be scanned from the first entry with a = 5 and b = 42 up through the last entry with a = 5. Index entries with c >= 77 would be skipped, but they'd still have to be scanned through. This index could in principle be used for queries that have constraints on b and/or c with no constraint on a — but the entire index would have to be scanned, so in most cases the planner would prefer a sequential table scan over using the index.
It does matter... slightly sometimes. There is something like data structure alignment. If you start using a mixture of types that are less than 4 bytes (smallint, char(1) etc), your table might get bigger if you interleave them. If you do use types that are at least 4 bytes, the order does not matter.
There is a good read https://www.2ndquadrant.com/en/blog/on-rocks-and-sand/
