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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.

Closes-Bug #1336983

Change-Id: I930fef13371d5d773a0df44601cf99a8a4dc8322

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