Have the tables used by the query been tuned? If not, I would suggest tuning the tables.
In the Management Portal, System Explorer -> SQL, switch to the proper namespace and then enter the query text in the Execute Query tab and press the Show Plan button. Now do the same with the query that selects just a single column and compare the query plans. Are they different?
If they are different, post the query plans here so we can have a look.
Possible other issues:
- Are any of the fields in the multi-field select list computed, and could the computation be taking a long time?
- Is the table using default storage, or %CacheSQLStorage? If %CacheSQLStorage, do any of the fields in the multi-select field list have expensive data retrieval logic?
- Are the tables in the from clause all base tables, or are views involved?
- Do the fields in the select list come from the same table?