pctfree only counts when BUILDING an index. It reserves X% of the block for subsequent
inserts into that block (inserts into the block can arise from inserts or UPDATES to the
base table).
All the pctfree does on an index is defer the splitting of an index block due to
subsequent inserts/updates AFTER the index is build. 10% is just a good "starting
point".
与表的pctfree值不同，对于索引来说，其pctfree仅仅是在create或rebuild时生效，对与后续的插入、修改之类的操作来说是无效的，pctfree的设置也仅仅是为了延缓由于insert等操作而导致的的索引块分裂。