has to do work to maintain indexes. If you insert into or delete
from a table, the system has to insert or delete rows in all the indexes on
the table. If you update a table, the system has to maintain those indexes
that are on the columns being updated. So having a lot of indexes can speed
up select statements, but slow down inserts, updates, and deletes.
Note: Updates and deletes with WHERE clauses can use indexes for scans,
even if the indexed column is being updated.