Xbase DBMS Chapter 8

From bfa452a375ea0a0a3f95304a69186936567e5263 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?J=C3=B6rg=20Frings-F=C3=BCrst?= Date: Mon, 14 Aug 2023 19:45:36 +0200 Subject: New upstream version 4.1.4 --- docs/html/xbc8.html | 79 ----------------------------------------------------- 1 file changed, 79 deletions(-) delete mode 100755 docs/html/xbc8.html (limited to 'docs/html/xbc8.html') diff --git a/docs/html/xbc8.html b/docs/html/xbc8.html deleted file mode 100755 index fcc461c..0000000 --- a/docs/html/xbc8.html +++ /dev/null @@ -1,79 +0,0 @@ - - -Xbase DBMS Chapter 8 - -

MDX Indices

Chapter Updated 12/02/22

- -The objective of this chapter is to provide information regarding the -basic concepts of how .MDX index files work in the Xbase environment.

- -The information in this chapter has been gathered by searching the internet -and by examining the structure of known good
- -

MDX Index File Characteristics

- -

MDX files are the same name as the corresponding DBF file with an MDX extension. -

MDX files are automatically opened by the library when the DBF file is opened. -

MDX index files (aka prod indices) contain from one to 47 tags, where each tag has it's own key characteristics. -

MDX indices maintain keys in either ascending or descending sort order. -

MDX indices support filtered keys. For example, a filter of .NOT. DELETED() will keep deleted records out -of the index tag. -

MDX indices are automatically updated by the Xbase library after the -indices are opened. - -

MDX indices support unique or non unique keys.

- -Unique keys must be unique if the UniqueKeyOption is not set to XB_EMULATE_DBASE. -If the UniqueKeyOption is set to XB_EMULATE_DBASE, then the database update routines will -add a record to the table, but not add a corresponding duplicate key to the index tag. -The UniqueKeyOption is off (don't allow duplicates) by default. -

- -Non-unique Keys are not required to be unique, duplicate -keys are allowed if the index is created with the XB_NOT_UNIQUE -setting. Duplicate keys are stored in record number order.

- - -

Character keys are left justified and padded on the right with spaces. -

Numeric keys are stored as twelve byte BCD values. -

Date keys are stored as eight byte double julian values. - -

MDX File Internals

- -The following information is not needed to use the library, it is just included -for general information.

- -MDX files are comprised of 512 pages where multiple pages make a block. The default -setting is 1024 blocks, each block containing two pages.

- -The first four pages contain: -

Bytes 0 - 543 contain general file information. -

Bytes 544 - 2047 is a 47 item table containing specific tag information. -

- -Pages five and beyound: -

Bytes 2048 and beyond contain tag header blocks, interior nodes and leaf nodes. - -

- -

Interior and Leaf Nodes

- -Interior Nodes and Leaf Nodes share the same structure in an NDX file with -the exception that interior nodes have a non zero number immediately -after the rightmost key on the node. - -Interior nodes point to other interior nodes or leaf nodes and leaf nodes point -to records in a DBF file. Interior nodes are optional nodes in an MDX file, -however if there are more than a few keys in the index there will -certainly be one or more interior nodes in the file. There will -always be at least one leaf node per tag in the file. Leaf nodes -contain DBF record numbers which point to the location of the record -in the DBF file.

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