Basic relationship: database---tablespace---data segment---partition---data block
Data segment (segment)
A segment (segment) consists of a series of extents. Usually a table is a segment.
Segments in Oracle can be divided into four types: data segments, index segments, rollback segments, and temporary segments.
The data segment is used to store user data, and each table has a corresponding rollback segment whose name is the same as that of the data table. The index segment is used to store system and user index information. The rollback segment is used to store the value before the user data is modified. The rollback segment has a one-to-many relationship with the transaction. A transaction can only use one rollback segment, and a rollback segment can store the rollback of one or more transactions. data. Temporary segments are used for ordering by order by statements as well as some aggregations.
SQL> desc dba_segments;
Name Type Nullable Default Comments
-------------------- ------------- ------- - ------- ------------------------------------------------------ -------------------------------------------------- ------------------------------------------
OWNER VARCHAR2(128) Y Username of the segment owner
SEGMENT_NAME VARCHAR2(128) Y Name, if any, of the segment
PARTITION_NAME VARCHAR2(128) Y Partition/Subpartition Name, if any, of the segment
SEGMENT_TYPE VARCHAR2(18) Y Type of segment: "TABLE", "CLUSTER", "INDEX", "ROLLBACK","DEFERRED ROLLBACK", "TEMPORARY","SPACE HEADER", "TYPE2 UNDO" or "CACHE"
SEGMENT_SUBTYPE VARCHAR2(10) Y SubType of Lob segment: "SECUREFILE", "ASSM", "MSSM", NULL
TABLESPACE_NAME VARCHAR2(30) Y Name of the tablespace containing the segment
HEADER_FILE NUMBER Y ID of the file containing the segment header
HEADER_BLOCK NUMBER Y ID of the block containing the segment header
BYTES NUMBER Y Size, in bytes, of the segment
BLOCKS NUMBER Y Size, in Oracle blocks, of the segment
EXTENTS NUMBER Y Number of extents allocated to the segment
INITIAL_EXTENT NUMBER Y Size, in bytes, of the initial extent of the segment
NEXT_EXTENT NUMBER Y Size, in bytes, of the next extent to be allocated to the segment
MIN_EXTENTS NUMBER Y Minimum number of extents allowed in the segment
MAX_EXTENTS NUMBER Y Maximum number of extents allowed in the segment
MAX_SIZE NUMBER Y Maximum number of blocks allowed in the segment
RETENTION VARCHAR2(7) Y Retention option for SECUREFILE segment
MINRETENTION NUMBER Y Minimum Retention Duration for SECUREFILE segment
PCT_INCREASE NUMBER Y Percent by which to increase the size of the next extent to be allocated
FREELISTS NUMBER Y Number of process freelists allocated in this segment
FREELIST_GROUPS NUMBER Y Number of freelist groups allocated in this segment
RELATIVE_FNO NUMBER Y Relative number of the file containing the segment header
BUFFER_POOL VARCHAR2(7) Y The default buffer pool to be used for segments blocks
FLASH_CACHE VARCHAR2(7) Y
CELL_FLASH_CACHE VARCHAR2(7) Y
INMEMORY VARCHAR2(8) Y Whether in-memory is enabled or not
INMEMORY_PRIORITY VARCHAR2(8) Y User defined priority in which in-memory column store object is loaded
INMEMORY_DISTRIBUTE VARCHAR2(15) Y How the in-memory columnar store object is distributed
INMEMORY_DUPLICATE VARCHAR2(13) Y How the in-memory column store object is duplicated
INMEMORY_COMPRESSION VARCHAR2(17) Y Compression level for the in-memory column store option
CELLMEMORY VARCHAR2(24) Y Cell columnar cache
oracle -- (three) data segment (segment)
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