Automated collection of necessary statistics When the optimizer requests a histogram for a given column, and that histogram does not exist, it may (depending on the cqds in effect) register a request for the histogram to be created at a later time, or utilize a small sample to generate a rudimentary histogram on the fly. In either case, when a subsequent Update Statistics statements specifies the ON NECESSARY COLUMNS clause, any column of the target table that has been the subject of one of these actions will have a bona fide histogram created.
Fix histograms for primary key of salted tables Users attempting to use Update Statistics to create a multi-column histogram (MC) corresponding to the primary key of a salted table may be unaware that the "_SALT_" column is implicitly prepended to the key as stated in the Create Table statement, and omit it. This fix will cause Update Stats to detect a request for a multi-column histograms that specifies the primary key columns (or a prefix of the full key), and add _SALT_ to it if missing, and order the MC to match the order of the columns in the primary key.
The change only affects salted tables, and is only applied if neither the ON EVERY KEY nor ON EVERY COLUMN clauses is present, because an MC matching the full primary key is automatically generated in those cases.
A second part of this fix applies to cases where ON EVERY KEY or ON EVERY COLUMN is specified in an Update Statistics statement on a salted table. By default, MCs corresponding to subsets of the primary key will no longer be generated automatically in this case. The cqd USTAT_ADD_SALTED_KEY_PREFIXES_FOR_MC may be set to 'ON' to cause MCs for subsets of the primary key to be generated.