Segment-Page Storage Functions

• local_segment_space_info(tablespacename TEXT, databasename TEXT)

  Description: Generates usage information about all extent groups in the tablespace.

  Return type:

  node_name	Node name
  extent_size	Extent specifications of an extent group. The unit is the number of blocks.
  forknum	Fork number
  total_blocks	Total number of extents in a physical file
  meta_data_blocks	Number of blocks occupied by the metadata managed in a tablespace, including the space header and map page but excluding the segment head
  used_data_blocks	Number of extents used for storing data, including the segment head
  utilization	Percentage of the number of used blocks to the total number of blocks, that is, (the value of used_data_blocks + the value of meta_data_block)/the value of total_blocks
  high_water_mark	High-water mark, indicating the number of allocated extents and maximum physical page number. Blocks that exceed the high-water mark are not used and can be directly recycled.

  Example:

  select * from local_segment_space_info('pg_default', 'postgres');
       node_name     | extent_size | forknum | total_blocks | meta_data_blocks | used_data_blocks | utilization | high_water_mark
  -------------------+-------------+---------+--------------+------------------+------------------+-------------+-----------------
   dn_6001_6002_6003 |           1 |       0 |        16384 |             4157 |                1 |     .253784 |            4158
   dn_6001_6002_6003 |           8 |       0 |        16384 |             4157 |                8 |     .254211 |            4165
  (2 rows)

• pg_stat_segment_extent_usage(int4 tablespace oid, int4 database oid, int4 extent_type, int4 forknum)

  Description: Specifies the usage information of each allocated extent in an extent group returned each time. extent_type indicates the type of the extent group. The value is an integer ranging from 1 to 5. If the value is not within the range, an error is reported. forknum indicates the fork number. The value is an integer ranging from 0 to 4. Currently, only the following values are valid: 0 for data files, 1 for FSM files, and 2 for visibility map files.

  Return type:

  Name	Description
  start_block	Start physical page number of an extent
  extent_size	Size of an extent
  usage_type	Usage type of an extent, for example, segment head and data extent
  ower_location	Object location of an extent to which a pointer points. For example, the owner of a data extent is the head of the segment to which the data extent belongs.
  special_data	Position of an extent in its owner. The value of this field is related to the usage type. For example, special data of a data extent is the extent ID in the segment to which the data extent belongs.

  The value of usage_type is enumerated. The meaning of each value is as follows:

  • Non-bucket table segment head: data segment head of a non-hash bucket table
  • Non-bucket table fork head: fork segment header of a non-segment-page table
  • Data extent: data block

  Example:

  select * from pg_stat_segment_extent_usage((select oid::int4 from pg_tablespace where spcname='pg_default'), (select oid::int4 from pg_database where datname='postgres'), 1, 0);
  start_block | extent_size |       usage_type       | ower_location | special_data
  -------------+-------------+------------------------+---------------+--------------
  4157 |           1 | Data extent |    4294967295 |            0
  4158 |           1 | Data extent |          4157 |            0

• local_space_shrink(tablespacename TEXT, databasename TEXT)

  Description: Compresses specified physical segment-page space on the current node. Only the currently connected database can be shrank.

  Return value: empty

• gs_space_shrink(int4 tablespace, int4 database, int4 extent_type, int4 forknum)

  Description: Works similar to local_space_shrink, that is, compresses specified physical segment-page space. However, the parameters are different. The input parameters are the OIDs of the tablespace and database, and the value of extent_type is an integer ranging from 2 to 5. Note: The value 1 of extent_type indicates segment-page metadata. Currently, the physical file that contains the metadata cannot be compressed. This function is used only by tools. You are not advised to use it directly.

  Return value: empty

• pg_stat_remain_segment_info()

  Description: Displays residual extents on the current node due to faults. Residual extents are classified into two types: segments that are allocated but not used and extents that are allocated but not used. The main difference is that a segment contains multiple extents. During reclamation, all extents in the segment need to be recycled.

  Return type:

  Name	Description
  space_id	Tablespace ID
  db_id	Database ID
  block_id	Extent ID
  type	Extent type. The options are as follows: ALLOC_SEGMENT, DROP_SEGMENT, and SHRINK_EXTENT.

  The values of type are described as follows:

  • ALLOC_SEGMENT: When a user creates a segment-page table and the segment is just allocated but the transaction of creating a table is not committed, the node is faulty. As a result, the segment is not used after being allocated.

  • DROP_SEGMENT: When a user deletes a segment-page table and the transaction is successfully committed, the bit corresponding to the segment page of the table is not reset and a fault, such as power failure, occurs. As a result, the segment is not used or released.

  • SHRINK_EXTENT: When a user shrinks a segment-page table and does not release the idle extent, a fault, such as power failure, occurs. As a result, the extent remains and cannot be reused.

    Example:

    select * from pg_stat_remain_segment_info();
    space_id | db_id | block_id | type
    ----------+-------+----------+------
    1663       |   16385|        4156| ALLOC_SEGMENT

• pg_free_remain_segment(int4 spaceId, int4 dbId, int4 segmentId)

  Description: Releases a specified residual extent. The value must be obtained from the pg_stat_remain_segment_info function. The function verifies input values. If the specified extent is not among the recorded residual extents, an error message is returned. If the specified extent is a single extent, the extent is released independently. If it is a segment, the segment and all extents in the segment are released.

  Return value: empty