Full table scans

The oracle optimizer uses full table scan in one of the following condition:

Lack of Index :  if query is unable to use the existing index, optimiser uses a full table scan ( unless a ROWID filter or cluster access path is available).

Large amount of data with low selectivity

Small table : A table which contains less than DB_FILE_MULTIBLOCK_COUNT_READ  blocks under high water mark. Full table is good for small tables.

Full Table Hints: If table is using FULL table hint.

High Degree of parallelism : Because of high degree parallelism optimizer uses full table scan over range scan.For more information on Degree of parallelism refer ALL_TABLES.