Table X (File architecture)

Stores all Record Extensions in a separate table from the the base records (so that Table B contains only these base records.

Enabled by reorganizing / CREATEing a file with XSIZE greater than 0.

Available as of Model 204 V7.1

Summary

The Table B of a Model 204 file has a maximum number of 16.7 million record numbers (IRNs). By default (XSIZE = 0), these 'slots' are used for both base records and extension records.

By implementing Table X (XSIZE > 0) all defined IRNs (up to 16.7 million) in Table B are used for base records only, allowing the storage of the maximum number of records in a file regardless of the number of extensions.

Table X is significantly larger than the 16.7 million limit of Table B; as the product of [[XSIZE parameter|XSIZE] * [[XRECPPG parameter|XRECPPG] may be up to 512 million extension slots.

When the defined capacity of Table B or Table X is exceeded the file is marked full or the updating transaction is backed out.

Table X may be implemented in one of two ways depending on the setting of the RECRDOPT parameter.

The default setting of the RECRDOPT (x'00')

If the default setting of RECRDOPT is used, while all extensions are stored in Table X, the record allocation in Table B is identical to what it is without Table X enabled.

BRECPPG remains the maximum number of records which may occur on a page, and so IRNs may still be wasted (never assigned to a record) as data fills up a page.

Setting RECRDOPT to x'01'

This setting changes the base record allocation to ensure that all defined IRNs are available for use.

For all x'01' RECRDOPT files, a base record size limit (the read only parameter BRLIMSZ) is calculated:

For an entry order file:

BRLIMSZ = (6140 / BRECPPG) - 2

For an unordered file:

BRLIMSZ = (6136 / BRECPPG) – 2

When a new record is stored, rather than using any and all space available on a page only up to BRLIMSZ) bytes will be stored in Table B. This ensures that the full [[BSIZE parameter|BSIZE] * [[BRECPPG parameter|BRECPPG] number of IRNs can actually be stored in the file.

(Note that the BRESERVE parameter is ignored with x'01' RECRDOPT files).

Advantages of Using Table X

Scalability

The obvious advantage of implementing Table X relates to the ability to store the full 16.7 million base records in a Model 204 file,

Indexing Performance

Even if a file has significantly fewer records than the limit, you should consider the performance of your indices in deciding whether to utilize Table X.