| 摘要 |
A database system is described that provides a methodology for significantly reducing latching costs associated with index pages in non-covered index scans, all without the need for using a conventional "shadow page." The database system uses a "look ahead" list (LA) to save up to a preset number (e.g., 60) of ROWIDs (row identifiers) from the qualifying index keys. Since a ROWID is typically small (e.g., about 6 bytes), the space needed to store 60 ROWIDs is typically much smaller than the underlying page size (which typically range from 2K to 32K in systems). Thus, instead of finding just one next qualifying key as is conventionally done, up to a selected number (e.g., 60) of qualifying keys on the current page are found and the ROWIDs from them are stored in the look ahead list. In many cases, the copying costs are eliminated too. In this manner, the system may avoid the substantial copying/latching overhead typically incurred for large scans for which several rows may be of interest.
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