// Macintosh programmers only, hack to allow setting types
extern long ctMacCreator;
extern long ctMacType;
CTREE BACKEND IMPLEMENTATION DETAILS
This provides some documentation of how the c-tree Plus backend works, as much
for our benefit as yours - you should never be fiddling with these mechanisms
unless you are prepared to accept the responsibility of possibly breaking the
database engine.
On the other hand, if you have suggested improvements - great!
COMPARATIVE SEARCHES FOR UNIQUE KEYS
Greater-than
Must append a suffix to the key that's the highest possible suffix.
eg:
key = 1
first actual value = 1 suffix 26,345
to prevent finding that key, we need to search for > 1 suffix MAXLONG
Greater-than or equal
Must append a suffix to the key that's the lowest possible suffix.
eg:
key = 1
first actual value = 1 suffix 26,345
to find that key, we need to search for > 1 suffix 0
Less than
Must append a suffix to the key that's the lowest possible suffix.
eg:
key = 1
first actual value = 1 suffix 26,345
to find that key, we need to search for < 1 suffix 0
Less than or equal
Must append a suffix to the key that's the highest possible suffix.
eg:
key = 1
first actual value = 1 suffix 26,345
to find that key, we need to search for < 1 suffix MAXLONG
DUPLICATE KEY BACKGROUND
c-tree handles dup keys by appending the record offset to the key, to make it unique. This means you can't predict the complete value of the key, or the starting key!
COMPARATIVE SEARCHES FOR DUPLICATE KEYS
Greater-than
Must append a suffix to the key that's the highest possible suffix.
eg:
key = 1
first actual value = 1 suffix 26,345
to prevent finding that key, we need to search for > 1 suffix MAXLONG
Greater-than or equal
Must append a suffix to the key that's the lowest possible suffix.
eg:
key = 1
first actual value = 1 suffix 26,345
to find that key, we need to search for > 1 suffix 0
Less than
Must append a suffix to the key that's the lowest possible suffix.
eg:
key = 1
first actual value = 1 suffix 26,345
to find that key, we need to search for < 1 suffix 0
Less than or equal
Must append a suffix to the key that's the highest possible suffix.
eg:
key = 1
first actual value = 1 suffix 26,345
to find that key, we need to search for < 1 suffix MAXLONG
There is a record buffer associated with every dbTable instance. (We can think about optimizing this with lazy evaluation later.)
This record buffer is mapped to the field by a table of offsets and lengths, stored in the OOFILE_tableBackend_ctree class. This mapping table is used in constructing the ctree ISEG structures describing the indexes. It is also used in transferring data between the record buffer and gui classes, or other operators that transfer data. (eg: operator=, operator char* & stream output).
Thus, most fields don't have a separate buffer but just refer into the main record buffer.
Blob, Text and other (future) expandable fields are contained in a separate file. There is one Blob file per connection (ie: superfile enclosing a set of c-tree files).
To optimize writing and reading Blob records, they are not pulled in from the database until needed, by some data interchange or linking to a gui editing class. The blobs are also only written back if updated. Thus, a blob is managed by a pair of length and record offset fields in the main record. The dbBlob object contains also a dirty flag and a pointer to a memory buffer.
A blob which has been cleared or never written has a zero length and no corresponding blob record. This means we have some special case handling for empty dbText fields, which contain a single char - the terminating null, but should be treated as zero-length blobs rather than length=1.
