This patch improves handling of NULLs in textual fields in ColumnStore.
Previously empty strings were considered NULLs and it could be a problem
if data scheme allows for empty strings. It was also one of major
reasons of behavior difference between ColumnStore and other engines in
MariaDB family.
Also, this patch fixes some other bugs and incorrect behavior, for
example, incorrect comparison for "column <= ''" which evaluates to
constant True for all purposes before this patch.
1. Add a new system variable, columnstore_use_cpimport_for_cache_inserts,
that when set to ON, uses cpimport for the cache flush into ColumnStore.
This variable is set to OFF by default. By default, we perform batch inserts
for the cache flush.
2. Disable DMLProc logging of the SQL statement text for the cache
flush operation in case of batch inserts. Under certain heavy loads
involving INSERT statements, this logging becomes a bottleneck for
the cache flush, causing subsequent inserts into the cache table to hang.
This changes the warning for truncation to the correct MariaDB error
code (1264).
In addition it passes the strict mode up into the DML class to roll back
correctly.
It also sets the abort_on_warning flag for updates as this isn't set on
the rnd_init phase but is needed for strict mode to work.
