Re: Porting Guide and Statement made about Char Vs VarChar

Why would the variable length and LOB data not be allocated contiguously? They are both variable length data; i.e. a 50MB LOB column may hold only 15K of data. A LOB could just as well map to the variable length /segment/, as opposed to yet another location [segment]. That is what the documentation suggests [with my own <edits>], that "Storage for LOB columns <are> allocated in the same manner as VARCHAR columns. When a column stored in the overflow storage area is referenced <either VARCHAR or LOB>, currently all of the columns in that area are paged into memory. A reference to a 'smaller' VARCHAR column that is in the overflow area can potentially force extra paging of LOB columns."

The database brings the /row/ into memory to avoid faulting. I do not specifically know how it works for the varlen segment, but I suppose the LOB data is at the end of the allocation, and it may only load to a specific page boundary to include all VARLEN data preceding it; i.e. to then allow a fault if/when the LOB is actually referenced. It is likely there is an algorithm which decides based on current storage management activity; e.g. an asynchronous request, which may or may not be satisfied before an actual reference, thus faulting only if the async I/O had not yet completed, and an async request to /purge/ the pages from memory could cancel any prior /bring/ into memory.

Regards, Chuck