Difference between revisions of "Table E and FILEORG X'100' files (File architecture)"

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Revision as of 13:04, 19 April 2013

Holds Large Object Data (BLOBs and CLOBs)


When FILEORG x'100'is set on a file, this is enabled by reorganizing / CREATEing a file with ESIZE greater than 0, and then defining one or more fields with an attribute of CLOB or BLOB (the file must have an ESIZE for the latter to occur).

Available as of Model 204 V7.5


Summary

Storage and manipulation of Large Objects (LOBs):

The 'chained LOBs' in Table E under FILEORG x'100' files has a number of advantages over the way LOBs are stored and accessed in non x'100' files.

*because the LOBs no longer need to be contiguous, space monitoring is simpler and files are less likely to fill up.

*fields may be defined so that 'small' entries may be stored 'locally' in the record (and so do not use an entire page in Table E)

*you no longer need to predict the fields may be defined so that 'small' entries may be stored 'locally' in the record (and so do not use an entire page in Table E)

Storing Large Objects

When a field is defined as a large object (LOB) (BLOB or CLOB attributes) a pointer is stored in the record (in Tables B or X), and, depending on the MINLOBE attribute and length of the value being stored, and the LOB value is stored in its proper place, as follows:


Pointer in the record

When you store a 'not null' Large Object value in Table E, a Large Object descriptor is stored in Table B (or X depending on where the field is being ADDed). The descriptor contains the length, and a pointer to the Large Object data in the Table E page and is 27 bytes long.

This is the same as non x'100' files.

Storing a Large Object

When the length of the value being stored is 0 (i.e.: a null string) the action taken depends on the 'repeating' type (AT-MOST-ONE, REPEATABLE and EXACTLY-ONE attributes). This is discussed in the next section.

Other than null strings, if the value being stored is less than or equal to the MINLOBE attribute for the field, then the value is stored directly in the record (Table B or X). Note that this is in addition to the pointer described above.

If the length of the value being stored is greater than the MINLOBE attribute for the field, the value is stored in a series of pages in Table E with the first (up to) 6140 bytes (4 bytes being reserved for the chain pointer) on the first page, the next (up to) 6140 bytes on the 'next' page, and so on. These pages are not, necessarily, contiguous. Any pages emptied by field or record deletes are first reused, before the pages at EHIGHPG are used. .

Null Strings and Large Objects

Unlike other field types, the STORE-DEFAULT (SD) and STORE-NULL (SN) attributes may not be set on LOB fields. This results in such fields having the equivalent to the LITeral option. The following table illustrates the results when null strings are stored in Large Object fields:

Attribute Explicit Null
fieldname = ' '
Implicit Null
fieldname = %X (where %X is null)
No reference in code
EXACTLY-ONE (Note 1) (Note 1) (Note 1)
AT-MOST-ONE field stored with null value not present not present
REPEATABLE field stored with null value not present not present

Note 1: the field will always act as if it is physically present (and return a null). However, exactly one fields react a bit differently depending on whether they are inside a Repeating Field Group (RFG) or not. Inside an RFG, all three operations will result in a field being present (as displayed in a PAI). Outside of RFGs, (and remembering that only the CHANGE statement may be used on exactly one fields) only the 'CHANGE fieldname TO %X' syntax will physically create an occurrence on the record.

The following Parameters and Commands relate to the Use of Table E

Note: Pages are either used or not in Table E, so there is no formula to determine whether a page is added to the reuse queue (unlike Tables B and X). Because of this, there are no EREUSE, EREUSED, or EQLEN parameters. The size of the reuse queue can always be determined by the difference between EHIGHPG and EPGSUSED.

EHIGHPG parameter The highest active Table E page. (The first page in Table E is page zero.)
EPGSUSED parameter The number of Table E pages currently in use.
ESIZE parameter The number of pages in Table E
FILEORG parameter The file organisation