Answer: If you run reindexing with CTX_DDL.SYNC_INDEX less often, your indexes will be less fragmented, and you will not have to optimize them as often.

However, your data becomes progressively more out-of-date, and that may be unacceptable to your users.

Overnight indexing is acceptable for many systems. In this case, data that is less than a day old is not searchable. Other systems use hourly, 10-minute, or 5-minute updates.

Answer: The best way is to time some queries, run index optimization, and then time the same queries (restarting the database to clear the SGA each time, of course). If the queries speed up significantly, then optimization was worthwhile. If they do not, then you can wait longer next time.

You can also use CTX_REPORT.INDEX_STATS to analyze index fragmentation.

Answer: Yes, the same way as for normal indexing. There are often far fewer records to be indexed during a synchronize operation, so it is not usually necessary to provide hundreds of megabytes of indexing memory.