Transforming default values dynamically

摘要

When altering records in a repository of information to add an attribute that has a non-NULL default value and a corresponding constraint to the records, no update record is issued and instead, the existing records are essentially treated as having a NULL value for the attribute being added. To compute a query over the records to which the attribute has been added, a value is generated for rows that hold NULL values for the attribute in which the value in which the NULL values held therein are translated to the default value.

主权项

1. A computer implemented method, comprising:
receiving an instruction that relates to defining an attribute within a body of records of a database wherein the instruction specifies a constraint on an attribute that restricts values in said attribute to non-NULL values and a non-NULL default value for said attribute; in response to receiving said instruction, adding said attribute to said body of records; wherein upon completing processing of said instruction:
said attribute holds NULL values for said body of records, andmetadata of said database defines:
said constraint on said attribute restricting said values in said attribute to said non-NULL values, andsaid non-NULL default value for said attribute; receiving a query to read said attribute; in response to receiving said query, computing said query, wherein computing said query includes:
making a determination that
(a) said metadata defines said constraint on said attribute restricting said values in said attribute to said non-NULL values,(b) said metadata defines said non-NULL default value for said attribute, and(c) two or more records of said body of records hold a NULL value for said attribute;returning, as a value for said two or more records, said non-NULL default value defined by said metadata instead of said NULL value; wherein the method is performed by one or more computing devices.