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.
Updates and deletes with WHERE clauses can use indexes for scans,
even if the indexed column is being updated.