DB tuning has many aspects:
1. When the DB is a SQL one (persistent classes) than as some said a "tune table might help, but this will help mostly with SQL queries, by optimizing the use of (correct) indices.
There are other SQL tools on the portal (query plan, sql index usage etc.) that might help to see if the indices are correct.

2. Using the correct data types for your data (e.g. using %Date data type to store dates in $horolog format is much more effective than a %String (YYYY-MM-DD) on large scaled databases, especially with indices where cache memory can not hold most of the DB or with systems with huge number of transactions).
BTW, this is true to both persistent classes (that you access qwith SQL) and "raw" globals that you access with COS (or other language).

3. In a large scald DBs if some of your queries are "heavy" - code optimization might also be a consideration (e.g. replacing SQL with COS that does direct $Order on the index globals).

quine ; quine routine is a routine that prints itself content
X "ZL "_$T(+0)_" ZP"
Q

to avoid hardcode the actual routine name, you may do:
X "ZL "_$T(+0)_" ZP"