Oracle logical database structure: data block

Changes that are performed on a data block are recorded in change vectors (see redo record).

The block size is operating system dependent and cannot be smaller than 2KB or larger than 32KB.

The block size determines the maximum key length of indexes (see ORA-01450: maximum key length (‥) exceeded).

The maximum key length of an index depends on the block size.

The database block address (DBA) identifies the location of a block in the storage system. It consists of a data file number and a block number.

The (hashed) value of the DBA is also used to determine the buffer cache's LRU list in which the block is referred (if the block is cached in the buffer cache).

dbms_utility provides the three functions make_data_block_address, data_block_address_block and data_block_address_file to convert between data block address and file/block number.

Although Oracle can store up to 1000 columns in a table or view (as of SCN), the maximum number of columns in a data block is limited to 255.

A direct path read reads a data block into the PGA rather than the buffer cache.

data block corruption

Logical structures

incremental backup

The init parameters

• db_block_checksum
• db_file_multiblock_read_count

ORA-01578 is the block corruption error and is recorded in the alert log.

The Oracle cluster object allows to store «related» records (for example with the same parent key) in the same database block.

The PL/SQL package dbms_rowid provides the some functions such as rowid_block_number, rowid_relative_fno and rowid_row_number which allow to determine the block number and the record number within this block of a given record, which is identified by its rowid.

Using alter system dump datafile … to dump a data block to a trace file which allows to inspect the data block's content.

Depending on the size of a LOB and its column's configuration (especially the enable/disable storage in row clause), LOB data is stored in the same data block as the record/row to which it belongs (in row storage) or in a special LOB segment.