Expression replacement in virtual columns

The present application is related to U.S. patent application Ser. No. 11/951,890, titled Virtual Columns, filed by Subhransu Basu and Harmeek Singh Bedi on Dec. 6, 2007 and U.S. patent application Ser. No. 11/951,933, titled Partitioning in Virtual Columns, filed by Subhransu Basu, Harmeek Singh Bedi, and Ananth Raghavan on Dec. 6, 2007, the entire contents of which are incorporated herein by reference.

The present invention relates to database systems, and in particular, to techniques for representing and manipulating expressions in database systems.

The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

FIG. 1 illustrates an example of a table containing two columns and a virtual column.

FIG. 2 is a diagram of a computer system that may be used in an implementation of an embodiment of the present invention.

In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention.

Introduction

In a database management system (DBMS), data is stored in one or more data containers, each container contains records, and the data within each record is organized into one or more fields. In relational DBMSs, the data containers are referred to as tables, the records are referred to as rows, and the fields are referred to as columns.

In a relational DBMS, many operations can be performed on columns in tables. For example, statistics can be gathered on columns. Columns can also be indexed. Furthermore, constraints can be defined on columns in different tables for the purpose of maintaining referential integrity between the tables. Columns are also utilized in performing join predicate push-down operations. Finally, tables can be partitioned on columns, enabling optimizations such as partition-pruning and partition-wise joins.

Data in columns is stored as part of a table in databases, consuming permanent storage resources. To circumvent the need to store some data, a user may choose to compute the data only when it is needed for an operation and discard the data once the operation has completed. This computation of data may be specified within an expression in a query language like SQL. Expressions provide a way to compute data on demand without the need for permanent storage.

Virtual Columns

An Example

According to one technique, virtual columns may be defined within a DBMS to facilitate the processing and manipulation of computed data. A virtual column is a derived, or computed, column, which may or may not be materialized on a disk. In other words, unlike a regular column, which is stored on a disk as part of a table, a virtual column may be stored for only a short period of time in temporary storage such as volatile memory, and may be discarded at the end of an operation. The values contained in a virtual column are derived by computation of an expression or function and are computed on demand. Virtual columns can be used in queries just like any other regular table columns, providing a simple, elegant, and consistent mechanism for accessing expressions in a SQL statement.

A virtual column is illustrated with the following DDL statement.

create table t 1 (c 1 number, c 2 number, c 3 as (c 1 +c 2 ))

When a DBMS receives and processes this statement, it defines and creates a table t 1 , where columns c 1 and c 2 contain values of the number data type. Table t 1 also includes a virtual column c 3 . When a DBMS creates a virtual column, such as c 3 , in response to receiving a DDL statement such as the one above, the DBMS generates metadata defining the virtual column as containing the results of an expression. A defined virtual column need not contain any physical data. Rather, a virtual column logically contains data values which are based on the results of an expression.

Virtual column c 3 is derived from the expression “c 1 +c 2 ”, where c 1 and c 2 are regular columns in table t 1 . The values in a virtual column conform to the data types in the underlying expression that describes the virtual column. In this case, because c 1 and c 2 contain values of the number data type, c 3 also necessarily contains values of the number data type. Table 100 in FIG. 1 illustrates an example illustrating a table t 1 with example values in regular columns c 1 and c 2 and corresponding computed values in virtual column c 3 . Table 100 contains five rows. In the first row, for example, column c 1 contains the value 1 and column c 2 contains the value 10 . Therefore, virtual column c 3 , which contains values that are sums of values in columns c 1 and c 2 , contains the value 11 in the first row. Although virtual column c 3 is shown as being in table t 1 , this is only for the purpose of illustration. Virtual column c 3 need not be stored on disk as part of table t 1 like regular columns c 1 and c 2 , and often will not be in order to conserve storage resources. When a query statement such as “select c 3 of t 1 ” is processed by a DBMS, the values of c 3 may be computed dynamically based on values stored in regular columns c 1 and c 2 at the time of the computation.

Once a virtual column is defined, it may be referenced in SQL queries like a regular column. For example, the following SQL statement uses the virtual column c 3 in a select statement.

select * from t 1 where c 3 >30

The above statement selects all rows from the table t 1 which contain a value greater than 30 in the c 3 column. Again, the values in c 3 are not stored on disk, but are computed on demand by a DBMS. Therefore, when a DBMS executes the above statement, the sums of the values in regular columns c 1 and c 2 are calculated and compared to the value 30 in determining which rows in t 1 fulfill the query request.

For simplicity, in the examples in this disclosure, virtual column c 3 is described by the expression c 1 +c 2 . Significantly, a virtual column may also be derived from SQL functions and user-defined functions in addition to expressions.

Replacing Expressions with Virtual Columns

Once defined, virtual columns may be utilized in the same fashion as regular columns, even though virtual columns are not necessarily materialized on disk. For example, in a DBMS, indexes can be created on virtual columns and column statistics can be gathered on virtual columns. Furthermore, a DBMS may enforce referential integrity between two database tables using virtual columns as primary keys. Virtual columns can also be considered by a query optimizer for join predicate push-downs like regular columns. Finally, a DBMS can partition a table using a virtual column as a partitioning key. Once partitioned, techniques such as partition pruning and partition-wise joins can then be employed on the partitioned tables.

Query statements processed by DBMSs, however, often do not expressly reference virtual columns. For example, pre-existing query programs may have been written before a DBMS supported virtual columns. Although query processing time for these programs would be significantly reduced if the programs are rewritten to take advantage of virtual columns, rewriting the programs incurs a high cost of human labor that often outweighs the advantage of increased query processing efficiency. Therefore, according to one technique, expressions in a query statement which are equivalent to defined virtual columns are automatically replaced with the equivalent virtual columns, allowing column-based optimizations for processing queries to be executed on the query statement without query rewrites by a human user. In one example, users of a particular database have written query programs without using virtual columns. The database administrator notices that many query statements in these query programs contain the expression c 1 +c 2 for a particular table, where c 1 and c 2 are regular columns of the particular table. Using DDL statements, the database administrator creates a virtual column c 3 in the particular table and defines it as c 1 +c 2 , but does not alter any of the query programs. In one embodiment, when the query programs are executed, the expression c 1 +c 2 is automatically replaced with virtual column c 3 . Once this replacement occurs, column functions such as statistics and join predicate push-downs can be performed by the DBMS on virtual column c 3 , thereby increasing the efficiency of the query execution.

According to one technique, the determination of equivalence is based on the logical equivalence between an expression in a query and an expression defining a virtual column. For example, if c 1 and c 2 are regular columns of the data type number, and if virtual column c 3 is defined as c 1 +c 2 , then both the expression c 1 +c 2 and the expression c 2 +c 1 are replaced with virtual column c 3 . In determining the logical equivalence of a first expression and a second expression, the associative, commutative, and distributive properties of the terms in the expression are taken into account. For example, if c 1 +c 2 are of a data type where c 1 +c 2 is not equal to c 2 +c 1 , then the expression c 2 +c 1 will not be replaced with virtual column c 3 .

Partial Expression Replacement

According to one technique, an expression may be partially replaced with a virtual column. For example, the query statement select * from t 1 where c 1 +c 2 +c 4 >30 may be replaced by the query statement select * from t 1 where c 3 +c 4 >30, if c 3 is a virtual column defined to be c 1 +c 2 .

Collisions in Expression Replacement

In some cases, an expression may be replaced by one of several candidate replacement expressions. In one example, a query statement includes the expression c 1 +c 2 +c 4 , where c 1 , c 2 , and c 4 are all regular columns in the table t 1 . Two virtual columns, c 3 and c 5 , have also been defined for table t 1 where c 3 is c 1 +c 2 and c 5 is c 2 +c 4 . In this example, the expression c 1 +c 2 +c 4 can thus be replaced with either the expression c 3 +c 4 or the expression c 1 +c 5 . According to one technique, when a base expression can be replaced by more than one replacement expression, the replacement expression with the least cost is selected. Cost analysis can be based on a variety of factors, such as the existence of indexes and materialized views for virtual columns. For example, if an index exists for c 5 but not c 3 , then the expression c 1 +c 5 is less costly than the expression c 3 +c 4 because values for c 5 need not be computed.

Hardware Overview

FIG. 2 is a block diagram that illustrates a computer system 200 upon which an embodiment of the invention may be implemented. Computer system 200 includes a bus 202 or other communication mechanism for communicating information, and a processor 204 coupled with bus 202 for processing information. Computer system 200 also includes a main memory 206 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus 202 for storing information and instructions to be executed by processor 204 . Main memory 206 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 204 . Computer system 200 further includes a read only memory (ROM) 208 or other static storage device coupled to bus 202 for storing static information and instructions for processor 204 . A storage device 210 , such as a magnetic disk or optical disk, is provided and coupled to bus 202 for storing information and instructions.

Computer system 200 may be coupled via bus 202 to a display 212 , such as a cathode ray tube (CRT), for displaying information to a computer user. An input device 214 , including alphanumeric and other keys, is coupled to bus 202 for communicating information and command selections to processor 204 . Another type of user input device is cursor control 216 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor 204 and for controlling cursor movement on display 212 . This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane.

The invention is related to the use of computer system 200 for implementing the techniques described herein. According to one embodiment of the invention, those techniques are performed by computer system 200 in response to processor 204 executing one or more sequences of one or more instructions contained in main memory 206 . Such instructions may be read into main memory 206 from another machine-readable medium, such as storage device 210 . Execution of the sequences of instructions contained in main memory 206 causes processor 204 to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software.

The term “machine-readable medium” as used herein refers to any medium that participates in providing data that causes a machine to operation in a specific fashion. In an embodiment implemented using computer system 200 , various machine-readable media are involved, for example, in providing instructions to processor 204 for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device 210 . Volatile media includes dynamic memory, such as main memory 206 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus 202 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications. All such media must be tangible to enable the instructions carried by the media to be detected by a physical mechanism that reads the instructions into a machine.

Common forms of machine-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punchcards, papertape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read.

Various forms of machine-readable media may be involved in carrying one or more sequences of one or more instructions to processor 204 for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 200 can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus 202 . Bus 202 carries the data to main memory 206 , from which processor 204 retrieves and executes the instructions. The instructions received by main memory 206 may optionally be stored on storage device 210 either before or after execution by processor 204 .

Computer system 200 also includes a communication interface 218 coupled to bus 202 . Communication interface 218 provides a two-way data communication coupling to a network link 220 that is connected to a local network 222 . For example, communication interface 218 may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface 218 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface 218 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

Network link 220 typically provides data communication through one or more networks to other data devices. For example, network link 220 may provide a connection through local network 222 to a host computer 224 or to data equipment operated by an Internet Service Provider (ISP) 226 . ISP 226 in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet” 228 . Local network 222 and Internet 228 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 220 and through communication interface 218 , which carry the digital data to and from computer system 200 , are exemplary forms of carrier waves transporting the information.

Computer system 200 can send messages and receive data, including program code, through the network(s), network link 220 and communication interface 218 . In the Internet example, a server 230 might transmit a requested code for an application program through Internet 228 , ISP 226 , local network 222 and communication interface 218 .

The received code may be executed by processor 204 as it is received, and/or stored in storage device 210 , or other non-volatile storage for later execution. In this manner, computer system 200 may obtain application code in the form of a carrier wave.

In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is the invention, and is intended by the applicants to be the invention, is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Any definitions expressly set forth herein for terms contained in such claims shall govern the meaning of such terms as used in the claims. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.