Memory tablespaces store memory-resident data and use checkpoint image files on disk to recover that data if needed. Space in a memory tablespace is allocated in 32KB pages that have a status of either free or used. When data is deleted from a table, the empty pages' status changes to free within that table but they are not returned to the tablespace until compaction. Truncating a table returns all its pages to the tablespace as free.

1. Memory Tablespaces
 Tablespaces in which memory-resident data is saved
Architecture (Memory TBS + Checkpoint Image Files)
log file#
log file#
log file#
log file#
log file#
Checkpoint Image-#-0 Checkpoint Image-#-1
Disk
Memory
Fuzzy / Ping-Pong Checkpointing
Memory TBS

2. Allocation of space in a memory tablespace
Space is allocated in 32KB page units
Page Status: Free or Used
Object Free Used
Tablespace
•Space that is not allocated to any
table
•Can be allocated to a particular table
in a page unit
•Space allocated to a particular table
•Cannot be used by any other table until the
table returns the page
Table
•Space that is allocated to a table but
contains no data
•Can be reused within that table
•Space that is allocated to a table and contains
data
•Cannot be reused unless the data is deleted

3. Changes to the Status of Pages
 When a DELETE statement is executed on a table
 The status of empty pages in the table changes from “Used” to “Free” (they
are reusable within that table).
 The pages are not returned to the tablespace.
 When compaction is performed after a DELETE statement, empty pages are
returned to the tablespace (they are usable by other tables) and their status
in the tablespace changes from “Used” to “Free”.
 When TRUNCATE TABLE is executed on a table
 All of the pages that were allocated to the table are returned to the
tablespace.
 The pages can be used by other tables, and their status in the tablespace
changes from “Used” to “Free”.
 When MOVE is executed on a table
 When data is transferred to another table using the MOVE statement, the
empty pages are reusable only in the table from which the data were moved
(same effect as DELETE execution).
 The pages are returned to the tablespace when compaction is later
performed.

4. How spaces are allocated
Create table T1
1 2 3 4
5 6 7 8
1
Insert data into T1
1
Insert data into T1
1 2
3
1
1 2 3
Memory Tablespace

5. How spaces are returned
1 2 3 4
5 6 7 8
Memory Tablespace
T1
1 2
3 4
21 3 4
65 7 8
COMPACTION on T1
1 2
3 4
INSERT data into T2
5 6
7 8
TRUNCATE on T1
1 2
3 4
DELETE on T1
3 4
5 6
7 8
3
4
3
4
1
2 3 4