Database: 3.1 Using Database Routines: General Rules to Follow

There are certain rules of "memory etiquette" you should follow when using DB files. For the most part, these rules are the same as the general rules of memory etiquette.

First and foremost, try to keep as few blocks locked as possible, and keep them locked for as short a time as possible. You should not usually need to keep more than one item locked at a time. If you need another item, unlock the first one first, even if they're in the same item block. (This will cost very little time since the item block is unlikely to be swapped to disk right away.) The main reason you should have two or more items open at once is if you are directly comparing them or copying data from one to another. In this case, you should unlock each item as soon as you're done with it.

Remember that items are implemented as chunks in LMem heaps. This means, for example, that when you allocate an item (or expand an existing one), the heap it resides in (i.e. the item block) may be compacted or moved on the global heap (even if it is locked). This will invalidate all pointers to items in that item block. As a general rule, you should not allocate (or expand) items if you have any items from that group locked. Do not allocate "ungrouped" items if you have any items from any of the "ungrouped" groups locked. If you must keep an item locked, keep track of the item's memory block and chunk handle so you can use DBDeref() to get the address again.

Finally, try to keep the blocks small. Most of this is done for you. When you allocate an item, the DB manager will put it in an uncrowded item block. If all item blocks are too large, it will allocate a new one. However, you should keep items from getting too large. If individual items get into the multi-kilobyte range, you should consider storing them a different way; for example, you could make each f the larger items a VM block or a VM chain.

GEOS SDK TechDocs|

3 Using Database Routines | Next:

3.2 Allocating and Freeing Groups