Julian Hyde created CALCITE-1991:
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Summary: In CREATE TABLE, allow generated columns (both virtual
and stored), CHECK constraints, and DEFAULT clause
Key: CALCITE-1991
URL: https://issues.apache.org/jira/browse/CALCITE-1991
Project: Calcite
Issue Type: Bug
Reporter: Julian Hyde
Assignee: Julian Hyde
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|https://en.wikipedia.org/wiki/Virtual_column].
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)|https://mariadb.com/kb/en/library/virtual-computed-columns/]";
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|https://docs.oracle.com/database/122/SQLRF/CREATE-TABLE.htm#SQLRF01402]";,
and the VIRTUAL keyword;
* DB2 has "[generated
columns|https://www.ibm.com/support/knowledgecenter/en/SSEPGG_11.1.0/com.ibm.db2.luw.sql.ref.doc/doc/r0000927.html|]"
which are always stored, and neither VIRTUAL nor STORED keyword.
Example:
{code}
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));
{code}
If neither {{VIRTUAL}} nor {{STORED}} is specified, {{VIRTUAL}} is the default.
You can insert into a generated column only if the expression is DEFAULT:
{code}
> 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.
{code}
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:
{code}
INSERT INTO Foo (i, o) VALUES (1, NULL)
{code}
To prevent NULL values, in other words to force {{o = i + 4}} to always be
TRUE, write the constraint as follows:
{code}
CREATE TABLE Foo(
i INTEGER,
o INTEGER,
CHECK ((o = i + 4) IS NOT FALSE))
{code}
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.
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