Sybase 12.4.2 manual Indexing tips, Using RAM disk, Picking the right index type, 452

Indexing tips

Operating systems use two different methods to allocate memory to disk cache: fixed and dynamic. A preset amount of memory is used in a fixed allocation; usually a 10–15 percent memory allocation is set aside. The operating system usually manages this workspace using a LRU (least recently used) algorithm. For a dynamic allocation, the operating system determines the disk cache allocation as it is running. The goal is to keep as much memory in active use as possible, balancing the demand for real memory against the need for data from disk.

Using RAM disk

When RAM disk is implemented, the operating system views a portion of memory as a disk drive. Disk operations involving RAM disk are very fast. When files are placed on RAM disk, the performance of processes using those files can improve dramatically. Primary candidates for RAM disks are programs and temporary files.

Warning! This is not recommended for database files or transaction log files.

Database integrity may be compromised if these files are placed on RAM disk.

Indexing tips

The following sections give some tips for selecting and managing indexes. See

Chapter 4, “Adaptive Server IQ Indexes” for more information on these topics.

Picking the right index type

It is important to pick the correct index type for your column data. Adaptive Server IQ provides some indexes automatically—a default index on all columns that optimizes projections, and an HG index for UNIQUE and single- column PRIMARY KEY columns. While these indexes are useful for some purposes, you need other indexes to process certain queries as quickly as possible. Adaptive Server IQ chooses the best index type for you when there are multiple index types for a column.

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