In CREATE TABLE, allow generated columns (both virtual and stored), CHECK constraints, and DEFAULT clause

Extend the CREATE TABLE statement added to the "server" module in CALCITE-707 with generated columns (both VIRTUAL and STORED), CHECK constraints, and DEFAULT clause.

Details:

• There are two kinds of generated columns: virtual and stored; we refer to the latter as "stored generated columns".
• Stored generated columns are calculated from an expression when a row is inserted into the table and stored in that table.
• Virtual columns are calculated from an expression when a row is read from a table.
• The DEFAULT clause provides a value when the column is not mentioned in the INSERT.
• Like columns with a DEFAULT, generated columns have an associated expression; but unlike columns with a DEFAULT, generated columns may not be specified in an INSERT statement (with a small exception involving the DEFAULT expression, described below).
• A CHECK constraint checks that a given condition is not false before inserting a row.
• When query involving a table with a stored generated column is planned, the planner sees a constraint as if there were a CHECK constraint on that column.

Comparison to other databases. Some databases confusingly use "virtual" as an umbrella term for both stored and non-stored:

• MySQL's "generated columns" may be declared either "stored" or "virtual", using the STORED and VIRTUAL keywords;
• MariaDB's "virtual (computed)" columns may be tagged VIRTUAL, PERSISTENT, STORED;
• MSSQL's "computed columns" may be virtual or persisted, with the PERSISTED keyword;
• Oracle only has "virtual columns", and the VIRTUAL keyword;
• DB2 has "generated columns" which are always stored, and neither VIRTUAL nor STORED keyword.

Example:

CREATE TABLE Foo (
  i INTEGER NOT NULL,
  j INTEGER,
  k INTEGER AS (i + 1), // virtual
  m INTEGER AS (i + 2) STORED, // stored generated
  n INTEGER DEFAULT (i + 4), // has default
  o INTEGER, // constrained
  CHECK (o = i + 4));

If neither VIRTUAL nor STORED is specified, VIRTUAL is the default.

You can insert into a generated column only if the expression is DEFAULT:

> INSERT INTO t (i, o) VALUES (1, 5);
Error: Cannot INSERT into generated column
> INSERT INTO t (i, o) SELECT empno, deptno FROM emp;
Error: Cannot INSERT into generated column
> INSERT INTO t (i, o) VALUES (1, DEFAULT);
OK.
> INSERT INTO t (i) VALUES (1);
OK.

We allow GENERATED ALWAYS before AS, and CONSTRAINT name before CHECK.

A CHECK constraint is satisfied if it evaluates to TRUE or UNKNOWN. Thus the following statement would insert one row:

INSERT INTO Foo (i, o) VALUES (1, NULL)

To prevent NULL values, in other words to force o = i + 4 to always be TRUE, write the constraint as follows:

CREATE TABLE Foo(
  i INTEGER,
  o INTEGER,
  CHECK ((o = i + 4) IS NOT FALSE))

Not part of this proposal:

• CHECK as part of a column definition;
• Invisible columns. Oracle has these, and they solve the problem that INSERT INTO t SELECT * FROM u fails if t has any generated columns;
• A variant of the DEFAULT clause that provides values when the incoming value is NULL.