Over the past fifteen years, empirical studies of the reference behavior of a number of database systems have produced seemingly contradictory results. The presence or absence of locality of reference and sequentiality have both been reported (or denied) in various papers. As such, the performance analyst or database implementor is left with little concrete guidance in the form of expected reference behavior of a database system under a realistic workload.We present empirical evidence that all of the previous results about database reference behavior are correct (or incorrect).That is, if the database reference sequence is viewed on a pertransaction instance or per-database basis, almost any reference behavior is discernible. Previous results which report the absolute absence or presence of a certain form of refer-. ence behavior were almost certainly derived from reference traces which were dominated by transactions or databases which exhibited a certain behavior. Our sample consists of roughly twenty-five million block references, from 350,000 transaction executions, directed at 175 operational on-line databases at two major corporations. As such, the sample is an order of magnitude more comprehensive than any other reported in the literature.We also present evidence that reference behavior is predictable and exploitable when viewed on a per-transaction basis or per-database basis. The implications of this predictability for effective buffer management are discussed. the database buffer. Slots in this buffer are frames into which physical blocks are placed to make data available for requesting programs.Data requests which cannot be satisfied with blocks residing in the buffer cause faults to occur. In effect, to manage a database buffer, the DBMS must implement block fetch, placement, and replacement policies which, at the conceptual level, are analogous to those required for virtual memory. Since only small portions of a database can normally be retained in the buffer area, servicing data requests requires the DBMS to routinely perform I/O operations.
Content-based retrieval of images is the ability t o retrieve images that are similar to a query image. Oracle8i Visual Information Retrieval provides this facility based on technology licensed from Virage, Inc. This product is built on top of Oracle8i interMedia which enables storage, retrieval and management of images, audios and videos. Images are matched using attributes such as color, texture and structure and e cient content-based retrieval is provided using indexes of an image index type. The design of the index type is based on a multi-level ltering algorithm. The lters reduce the search space so that the expensive comparison algorithm operates on a small subset of the data. Bitmap indexes are used to evaluate the rst lter resulting in a design which performs well and is scalable. The image index type is built using Oracle8i extensible indexing technology, allowing users to create, use, and drop instances of this index type as they would any other standard index. In this paper we present an overview of the product, the design of the image index type, and some performance results of our product.
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