Merging synopses to determine number of distinct values in large databases
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United States of America Patent
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Oct 13, 2009
Grant Date -
May 22, 2008
app pub date -
Apr 25, 2007
filing date -
Nov 16, 2006
priority date (Note) -
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status (Latency Note)
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Abstract
A method and apparatus for merging synopses to determine a database statistic, e.g., a number of distinct values (NDV), is disclosed. The merging can be used to determine an initial database statistic or to perform incremental statistics maintenance. For example, each synopsis can pertain to a different partition, such that merging the synopses generates a global statistic. When performing incremental maintenance, only those synopses whose partitions have changed need to be updated. Each synopsis contains domain values that summarize the statistic. However, the synopses may initially contain domain values that are not compatible with each other. Prior to merging the synopses the domain values in each synopsis is made compatible with the domain values in the other synopses. The adjustment is made such that each synopsis represents the same range of domain values, in one embodiment. After “compatible synopses” are formed, the synopses are merged by taking the union of the compatible synopses.
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