什么时候应该使用 PostgreSQL 索引包含？

Question

Assume that I have a query like below:

Select 
t2.date,
max(t1.title),
sum(t2.spend)
...
from table1 t1
left join table t2 on t1.id = t2.t1_id
where t1.sup_id = 1
group by t2.date
order by t2.date

PostgreSQL 11+ supports index include. But I couldn't understand when we should use it. Could you please show it on this example or another simple example?

Answer 1

From documentation:

The optional INCLUDE clause specifies a list of columns which will be included in the index as non-key columns. A non-key column cannot be used in an index scan search qualification, and it is disregarded for purposes of any uniqueness or exclusion constraint enforced by the index. However, an index-only scan can return the contents of non-key columns without having to visit the index's table, since they are available directly from the index entry. Thus, addition of non-key columns allows index-only scans to be used for queries that otherwise could not use them.

In your case imagine you have an index on sup_id. If you only select sup_id in your query postgres will do an index only scan, but if you select other columns along with id, such as title and date, postgres will have to visit the index table and select these columns, which is slower than an index-only scan.
So if you want your select query to be faster you include the columns you want in your index. Beware that this will increase the size of the index considerably. Only use it if you have a small number of columns in your select query.