index behavior question - multicolumn not consulted ?

[Jonathan Vanasco]

I have a table with over 1MM records and 15 columns.

I had created a "unique index" on a mix of two columns to enforce a constraint : (resource_type_id, lower(archive_pathname))

i've noticed that searches never use this. no matter what I query, even if it's only the columns in the index. I'm seeing a 550ms sequential scan on everything.

If I create an index only on the text field: lower(archive_pathname) , all the queries use that and complete in 1.4ms

does anyone know why this happens ?

[John R Pierce]

Post by Jonathan Vanasco
I had created a "unique index" on a mix of two columns to enforce a constraint : (resource_type_id, lower(archive_pathname))
i've noticed that searches never use this. no matter what I query, even if it's only the columns in the index. I'm seeing a 550ms sequential scan on everything.
If I create an index only on the text field: lower(archive_pathname) , all the queries use that and complete in 1.4ms
does anyone know why this happens ?

the index (resource_type_id, lower(archive_pathname)) should certainly
get used on queries like...

select <stuff> from table where resource_type_id = $1 and
lower(archive_pathname) = $2; the 2nd = could be > or < too.

It also should get used on queries with resource_type_id = $1, unless
thats not very selective (millions of records with just a few
resource_type_id values, for instance).
--
john r pierce 37N 122W
somewhere on the middle of the left coast