Table E (File architecture): Difference between revisions

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Enabled by reorganizing / [[CREATE command: File|CREATE]]ing a file with [[ESIZE parameter|ESIZE]] greater than 0, and then [[DEFINE FIELD command|defining a field]] with a datatype of CLOB or BLOB (the file must have an ESIZE for the latter to occur).  
Enabled by reorganizing / [[CREATE command: File|CREATE]]ing a file with [[ESIZE parameter|ESIZE]] greater than 0, and then [[DEFINE FIELD command|defining a field]] with a data type of CLOB or BLOB (the file must have an ESIZE for the latter to occur).  


Available as of Model 204 V7.1  
Available as of Model 204 V7.1  

Revision as of 20:43, 2 May 2013

Holds Large Object Data (BLOBs and CLOBs)


Enabled by reorganizing / CREATEing a file with ESIZE greater than 0, and then defining a field with a data type of CLOB or BLOB (the file must have an ESIZE for the latter to occur).

Available as of Model 204 V7.1


Summary

Storage and manipulation of Large Objects (LOBs) differs greatly depending on whether FILEORG x'100' is set.

Table E without FILEORG x'100'
Table E with FILEORG x'100'

Regardless of the FILEORG setting, an individual LOB occurrence may be up to 2 Gb. The maximum size of Table E is 1,073,741,823 pages.


Table E Differences Without / With the x'100' Setting

Function non x'100' x'100'
General space management LOBs require contiguous space

New LOBs are always written starting at EHIGHPG; if ESIZE is reached, Table E is scanned for contiguous space (due to deletes).

Table E can 'fill' even with many available pages if the space is fragmented.
LOBs are stored as a series of chained pages, and new LOBs are written first using those in the reuse queue (due to deletes).

Table E will 'fill' only if all pages are used.
Overhead There is a 27 byte pointer in the record on Table B / X for every occurrence of every LOB

There are bitmap pages for every 49152 pages in Table E (plus 16 additional overhead pages)

The first page of every LOB occurrence contains a 32 byte LOB header.
There is a 27 byte pointer in the record on Table B / X for every occurrence of every LOB

There are 4 bytes reserved on every page in Table E for the chaining of LOBs.
Storing small amounts of data in a LOB field Even a single byte of data requires a full page of Table E Fields may be defined with the MINLOBE attribute to permit small (MINLOBE may be set up to 200 bytes) of data to be stored directly in the record in Table B. But see the 'Change' section, below.
Change a LOB At the time a LOB is first stored, if you expect to subsequently need to increase its size, you will need to RESERVE the necessary space in the initial add. LOBs may be changes as needed (on adds and stores, the RESERVE, if present, is ignored).

Change statements are not permitted if MINLOBE is in use. (A delete / add or insert may be used).

COMPACTE command Is used to defragment Table E to increase the amount of contiguous space. The COMPACTE command is not necessary, and attempts to execute it will generate an error.