Validating an index
How can you tell when an index would benefit from being rebuilt?
There are two Oracle views that provide index statistics, DBA_INDEXES and INDEX_STATS.
The SQL in listing 8.3 will perform an Oracle analyze index xxx validate structure for each index in the schema and report of the resulting values in the Oracle index_stats.
Note: Running id1will invoke id2through id5automatically producing the unbalanced index report.
The script id1provides a method for getting a complete report for all indexes.
In all the examples presented here, we’ll apply k-means, PAM and hierarchical clustering.Recall that the goal of partitioning clustering algorithms (Part @ref(partitioning-clustering)) is to split the data set into clusters of objects, such that: In this section, we’ll describe the two commonly used indices for assessing the goodness of clustering: the silhouette width and the Dunn index.These internal measure can be used also to determine the optimal number of clusters in the data.For each observation If the data set contains compact and well-separated clusters, the diameter of the clusters is expected to be small and the distance between the clusters is expected to be large. The aim is to compare the identified clusters (by k-means, pam or hierarchical clustering) to an external reference.It’s possible to quantify the agreement between partitioning clusters and external reference using either the corrected It can be seen that several samples, in cluster 2, have a negative silhouette coefficient. We can find the name of these samples and determine the clusters they are closer (neighbor cluster), as follow: All the above elements can be used to evaluate the internal quality of clustering.