Bitmap and B-Tree

Bitmap indexes are most suitable for columns having very few unique values (very low cardinality). This is best for multiple combination of low to medium cardinality columns.
B-tree indexes are appropriate for medium to high cardinality columns having many distinct values.
Bitmap indexes are more advantageous than btree when the table has million of rows and the key column has low cardinality. bitmap indexes provide better performance than b tree indexes when queries often use a combination of multiple where conditions involving the OR operator, the table is read-only, or when there is low update activity on the key columns.

Coalescing

True statement regarding space management is:
In a locally managed tablespace free space are managed by bitmap
Coalescing of free space is not required in a locally managed tablespace
n.b.: Coalescing free space is not necessary for locally managed tablespaces, because bitmaps automatically track adjacent free space.
But free space in a dictionary managed tablespace can become fragmented, making it difficult to allocate new extents. For this way coalescing of free space is needed in a dictionary managed tablespace.
SQL > ALTER TABLESPACE tabsp_4 COALESCE;
Coalescing operation reduces one of the index partitions, which are partitioned using hash method. This method redistributes the index entries of an indexed partition.
SQL > ALTER INDEX parts1_hgidx COALESCE PARTITION PARALLEL