Please any members from the community can help me in understanding the conditions used in the query.

select a.*, round(index_leaf_estimate_if_rebuilt/current_leaf_blocks*100) percent, case when index_leaf_estimate_if_rebuilt/current_leaf_blocks < 0.5 then 'candidate for rebuild' end status

from

(

select table_name, index_name,owner, current_leaf_blocks, round (100 / 90 * (ind_num_rows * (rowid_length + uniq_ind + 4) + sum((avg_col_len) * (tab_num_rows) ) ) / (8192 - 192) ) as index_leaf_estimate_if_rebuilt

from (

select tab.table_name,tab.owner, tab.num_rows tab_num_rows , decode(tab.partitioned,'YES',10,6) rowid_length , ind.index_name, ind.index_type, ind.num_rows ind_num_rows, ind.leaf_blocks as current_leaf_blocks,

decode(uniqueness,'UNIQUE',0,1) uniq_ind,ic.column_name as ind_column_name, tc.column_name , tc.avg_col_len