Partial Indexes: Indexing Selected Rows
So far we have only discussed which columns to add to an index. With partial (PostgreSQL) or filtered (SQL Server) indexes you can also specify the rows that are indexed.
A partial index is useful for commonly used where conditions that use constant values—like the status code in the following example:
SELECT message FROM messages WHERE processed = 'N' AND receiver = ?
Queries like this are very common in queuing systems. The query fetches all unprocessed messages for a specific recipient. Messages that were already processed are rarely needed. If they are needed, they are usually accessed by a more specific criteria like the primary key.
We can optimize this query with a two-column index. Considering this query only, the column order does not matter because there is no range condition.
CREATE INDEX messages_todo ON messages (receiver, processed)
The index fulfills its purpose, but it includes many rows that are never searched, namely all the messages that were already processed. Due to the logarithmic scalability the index nevertheless makes the query very fast even though it wastes a lot of disk space.
With partial indexing you can limit the index to include only the unprocessed messages. The syntax for this is surprisingly simple: a where clause.
CREATE INDEX messages_todo ON messages (receiver) WHERE processed = 'N'
The index only contains the rows that satisfy the where clause. In this particular case, we can even remove the
PROCESSED column because it is always
'N' anyway. That means the index reduces its size in two dimensions: vertically, because it contains fewer rows; horizontally, due to the removed column.
The index is therefore very small. For a queue, it can even mean that the index size remains unchanged although the table grows without bounds. The index does not contain all messages, just the unprocessed ones.
The where clause of a partial index can become arbitrarily complex. The only fundamental limitation is about functions: you can only use deterministic functions as is the case everywhere in an index definition. SQL Server has, however, more restrictive rules and neither allow functions nor the
OR operator in index predicates.
A database can use a partial index whenever the where clause appears in a query.
Think About It
What peculiarity has the smallest possible index for the following query:
SELECT message FROM messages WHERE processed = 'N';