1. Code simplification:
Item_default_value handled all these values:
a. DEFAULT(field)
b. DEFAULT
c. IGNORE
and had various conditions to distinguish (a) from (b) and from (c).
Introducing a new abstract class Item_contextually_typed_value_specification,
to handle (b) and (c), so the hierarchy now looks as follows:
Item
Item_result_field
Item_ident
Item_field
Item_default_value - DEFAULT(field)
Item_contextually_typed_value_specification
Item_default_specification - DEFAULT
Item_ignore_specification - IGNORE
2. Introducing a new virtual method is_evaluable_expression() to
determine if an Item is:
- a normal expression, so its val_xxx()/get_date() methods can be called
- or a just an expression substitute, whose value methods cannot be called.
3. Disallowing Items that are not evalualble expressions in table value
constructors.
A CTE can be defined as a table values constructor. In this case the CTE is
always materialized in a temporary table.
If the definition of the CTE contains a list of the names of the CTE
columns then the query expression that uses this CTE can refer to the CTE
columns by these names. Otherwise the names of the columns are taken from
the names of the columns in the result set of the query that specifies the
CTE.
Thus if the column names of a CTE are provided in the definition the
columns of result set should be renamed. In a general case renaming of
the columns is done in the select lists of the query specifying the CTE.
If a CTE is specified by a table value constructor then there are no such
select lists and renaming is actually done for the columns of the result
of materialization.
Now if a view is specified by a query expression that uses a CTE specified
by a table value constructor saving the column names of the CTE in the
stored view definition becomes critical: without these names the query
expression is not able to refer to the columns of the CTE.
This patch saves the given column names of CTEs in stored view definitions
that use them.
query with VALUES()
A table value constructor can be used in all contexts where a select
can be used. In particular an ORDER BY clause or a LIMIT clause or both
of them can be attached to a table value constructor to produce a new
query. Unfortunately execution of such queries was not supported.
This patch fixes the problem.
a table value constructor shows wrong number of rows
This is another attempt to fix this bug. The previous patch did not take
into account that a transformation for ALL/ANY subqueries could be applied
to the materialized table that wrapped the table value constructor used as
a specification of the subselect used an ALL/ANY subquery. In this case
the result of the derived table used a sink of the class select_subselect
rather than of the class select_unit. Thus the previous fix could cause
memory overwrites when running EXPLAIN for queries with table value
constructors in ALL/ANY subselects.
This patch always provides columns of the temporary table used for
materialization of a table value constructor with some names.
Before this patch these names were always borrowed from the items
of the first row of the table value constructor. When this row
contained expressions and expressions were not named then it could cause
different kinds of problems. In particular if the TVC is used as the
specification of a derived table this could cause a crash.
The names given to the expressions used in a TVC are the same as those
given to the columns of the result set from the corresponding SELECT.
value constructor shows wrong number of rows
If the specification of a derived table contained a table value constructor
then the optimizer incorrectly estimated the number of rows in the derived
table. This happened because the optimizer did not take into account the
number of rows in the constructor. The wrong estimate could lead to choosing
inefficient execution plans.
