A full scan is available if a predicate references one of the columns in the index. The
predicate does not need to be an index driver. A full scan is also available when
there is no predicate, if both the following conditions are met:
n All of the columns in the table referenced in the query are included in the index.
n At least one of the index columns is not null.
A full scan can be used to eliminate a sort operation, because the data is ordered by
the index key. It reads the blocks singly.
Fast Full Index Scans
Fast full index scans are an alternative to a full table scan when the index contains
all the columns that are needed for the query, and at least one column in the index
key has the NOT NULL constraint. A fast full scan accesses the data in the index
itself, without accessing the table. It cannot be used to eliminate a sort operation,
because the data is not ordered by the index key. It reads the entire index using
multiblock reads, unlike a full index scan, and can be parallelized.
Fast full scan is available only with the CBO. You can specify it with the
initialization parameter OPTIMIZER_FEATURES_ENABLE or the INDEX_FFS hint.
Fast full index scans cannot be performed against bitmap indexes.
A fast full scan is faster than a normal full index scan in that it can use multiblock
I/O and can be parallelized just like a table scan.
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index full scan需要先从root定位到第一个leaf block，然后按顺序一个一个读取所有的leaf block，所以index full scan可以用来避免某些sort操作，这个full scan的名字有点误导人，其实并不是所有的index block都被读取的，某些分支块是不会读到的
index fast full scan则读取index的所有block，包括branch block，并且是multiblock的读取方式，所以index fast full scan不能用来消除sort ;
ffs只能在cbo下使用，我想主要原因是并行的关系 .
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1. 是避免排序，因为索引已经排序，不是读取索引的全部块。通过链接读取下一块。
2。Fast Full Index Scans，全部读取，包括根，叶等结点。
充分发挥多块读的特性。
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