Schema snapshot isolation access in a relational database

A method in a relational database system, the method comprising:

• receiving a data read command comprising first and second queries, each query referencing a metadata object in a metadata storage of the relational database system and configured to read user data objects from a user data storage of the relational database system;
• receiving a metadata modification command to be executed concurrently with the data read command, and wherein execution of the metadata modification command causes the metadata object to be modified;
• in response to receiving the data read command:
• starting a first transaction at a time T1;
• obtaining a metadata access lock from a lock manager of the relational database system;
• creating a first query plan for the first query, wherein the first query plan references the metadata object;
• beginning execution of the first query plan inside the first transaction at a time T2 using the metadata access lock;
• completing execution of the first query plan at a time T3;
• and
• committing the first transaction at a time T4;
• and
• in response to receiving the metadata modification command:
• starting a second transaction concurrently with the first transaction at a time T5;
• obtaining a metadata change lock from the lock manager;
• modifying the metadata object inside the second transaction using the metadata change lock, wherein the modification is not blocked by the metadata access lock;
• and
• committing the second transaction at a time T6;
• wherein said time T2 is subsequent to said time T1, said time T5 is subsequent to said time T2, said time T3 is subsequent to said time T5, and said time T6 is subsequent to said time T3.

A computer program product comprising a computer-readable memory device having computer program logic recorded thereon that when executed by at least one processor of a computing device causes the at least one processor to perform operations, the operations comprising:

• receiving a data read command comprising first and second queries, each query referencing a metadata object in a metadata object in a metadata storage of the relational database system and configured to read user data objects from a user data storage of the relational database system;
• receiving a metadata modification command to be executed concurrently with the data read command, and wherein execution of the metadata modification command causes the metadata object to be modified;
• in response to receiving the data read command:
• starting a first transaction at a time T1;
• obtaining a metadata access lock from a lock manager of the relational database system;
• creating a first query plan for the first query, wherein the first query plan references the metadata object;
• begin executing the first query plan inside the first transaction at a time T2 using the metadata access lock;
• completing execution of the first query plan at a time T3;
• and
• committing the first transaction at a time T4;
• and
• in response to receiving the metadata modification command:
• starting a second transaction concurrently with the first transaction at a time T5;
• obtaining a metadata change lock;
• modifying the metadata object inside the second transaction using the metadata change lock, wherein the modification is not blocked by the metadata access lock;
• and
• committing the second transaction at a time T6,
• wherein said time T2 is subsequent to said time T1, said time T5 is subsequent to said time T2, said time T3 is subsequent to said time T5, and said time T6 is subsequent to said time T3.