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Do people usually make every column in a table a secondary index to be on the safe side in case the customer decides to use either field to search for a record?
Does the search first go through the secondary indexes and then to the primary key? Thus narrowing down to the requested data?
What is the point of having secondary index if you already have a column that is a primary key?
Usually you only index columns that need to be. Adding additional indexes would normally be considered premature optimization.
Most optimizers will identify the fastest method to find the least number of records. This may be to use and index, but may be a full table scan. If there are multiple indexes that can be used, often only one is used, and the resulting records compared against the remaining criteria. If multiple indexes are used, then the resulting result sets need to be matched, and records which weren’t found in both indexes eliminated.
It is common to use surrogate keys for tables where the natural key is subject to change, or very (purposely vague) long. The natural key in this case would be indexed as a secondary unique key. In some cases there may be competing natural keys, in which case all the natural keys would have unique indexes.