In oracle 10g's data dictionary, what is the difference between user_tables and user_segments?

Question

when querying both these tables, i get a different number of blocks for a particular table in each. also the number of rows in user_tables differs from the actual number of rows in a table (got from select * query)

Answer 1

I'll have to think about the first part. But, the number of rows in a table in user_tables is updated when you analyze the table. So, the actual number in the table being different simply means that rows have been inserted or deleted since the last analyze. This is nothing to worry about unless the discrepancy is a in terms of a very high percentage. Then, the optimizer may not always select the best access method. For instance, if it thinks there are only a few rows, it may decide a full table scan is quicker. If the table actually has millions of rows, it will kill performance and an index access should have been used.