Method for concurrently performing a physical sequential scan of a database into a database buffer which is queued until a preceding scan is completed

• US 5,717,918 A
• Filed: 06/06/1995
• Issued: 02/10/1998
• Est. Priority Date: 06/17/1994
• Status: Expired due to Term

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: Takayama, Hiroshi, Yamashita, Kuniaki, Masai, Kazuo, Miyazaki, Mitsuo
• Primary Examiner(s): Black, Thomas G.
• Assistant Examiner(s): RONES, CHARLES
• Application Number:
  US08/467,278
• Time in Patent Office: 980 Days
• Field of Search: 395/425, 395/600, 395/250, 395/872, 395/650, 395/312, 395/492, 395/608, 395/618, 395/620, 364/200
• US Class Current: 1/1
• CPC Class Codes: G06F 16/24557 Efficient disk access during query execution
• CPC Class Codes: Y10S 707/99938 Concurrency, e.g. lock management in shared database
• CPC Class Codes: Y10S 707/99953 Recoverability

Abstract

A database system is provided for reading data from a database to a database buffer and performing a physical sequential scan for the data on the buffer. If the preceding request for physical sequential scan is under execution, the database system operates to keep the succeeding request for scan in a waiting state until the preceding requested scan is completed and then execute the succeeding request for scan after the preceding requested scan is completed. If there are more than one succeeding requests for physical sequential scan, they are executed at the same time. Another scanning method is also provided. If the preceding request for scan is under execution, the succeeding requested scan is executed for the data existing after the halfway location at which the preceding scan process now reads the data. The short data is read from its head for completing the subsequent scan.

Claims

1. A scanning method executed in a database processing device for concurrently executing plural scan processes for one database, comprising the steps of:

determining whether or not a preceding physical sequential scan is being executed for said database if a request for physical sequential scan is issued to said database;

if said preceding scan is being executed, keeping the succeeding request for physical sequential scan in a waiting state until said preceding scan is completed; and

executing the succeeding request for physical sequential scan after said preceding scan is completed.

2. A scanning method as claimed in claim 1, wherein if the succeeding plural requests for physical sequential scan are awaited, those requested scans are started at a time after said preceding requested scan is completed.

3. A scanning method as claimed in claim 2, wherein said database processing device provides buffer memories for temporarily storing data read out of said database, sand data is used for said scan, and said plural physical sequential scans are executed by using said buffer memories for storing corresponding data units in said database.

4. A scanning method as claimed in claim 3, wherein said scan process is a collating process without changing the change of said database.

5. A scanning method as claimed in claim 3, executed in plural database processing devices connected through a communication line, comprising the steps of:

checking for an empty area in a buffer memory of another database processing device if stored data is discarded from said buffer memory;

transferring said data to be discarded to said empty area if any; and

giving a request for scan to one of said database processing device for holding said data if said data discarded from the original database processing device is requested to be re-scanned.

6. A scanning method for executing plural scans for one database in a database processing device arranged to temporarily store each predetermined data unit from said database to a buffer memory and execute said scan by using said stored data, comprising the steps of:

determining whether or not a preceding physical sequential scan is now being executed for said database if a physical sequential scan for said database takes place;

storing a location of said database being processed in said scan process under execution if any;

starting the succeeding scan process from said stored location in parallel to said preceding physical sequential scan; and

executing said succeeding scan process for said database from its head to said stored location after said preceding physical sequential scan is completed.

7. A scanning method as claimed in claim 6, wherein said scan process is a retrieving process with no change of said database.

8. A scanning method as claimed in claim 6, executed in plural database processing devices connected with one another through a communication line, further comprising the steps of:

checking for an empty area of said buffer memory of another database processing device if said stored data is discarded from said buffer memory; and

if said empty area is found, transferring said data to be discarded to said empty area.

9. A scanning method as claimed in claim 8, wherein if said data discarded from the original database processing device is requested to be rescanned, a request for scan is issued to a database processing device for holding said data.

10. A scanning method as claimed in claim 8, wherein if said data discarded from the original database processing device is requested to be rescanned, said data is transferred from a database processing device for holding said data to said buffer memory of the original database processing device.

11. A scanning method executed in a database system having database processing devices arranged to store each predetermined unit data from a database to the corresponding buffer memories and perform the scan for said stored data and a communication line for connecting said database processing devices, comprising the steps of:

in any one of said database processing devices,

if a request for scanning said database is issued, checking whether or not the target data stays in the buffer memory of another database processing device;

if the target data stays in said buffer memory, issuing an execution instruction for scanning said database to another database processing device; and

if no target data stays in said buffer memory, reading each predetermined data unit from said database to said buffer memory of another database processing device and executing said scan process.

12. A scanning method executed in a database system having database processing devices arranged to read each predetermined data unit from a database to the corresponding buffer memories and perform a scan process for said read data and a communication line for connecting said database processing devices, comprising the steps of:

in any one of said database processing devices,

if a request for scanning said database takes place, checking whether or not target data stays in the buffer memory of another database processing device;

if said target data stays in said buffer memory, transferring said data to said buffer memory of another database processing device and executing a scan process for said data; and

if no target data stays in said buffer memory, reading each predetermined data unit from said database to said buffer memory and executing a scan process for said data.

13. A scanning method executed in a database system having a plurality of database processing devices and a communication line for connecting the database processing devices, a database being stored in an external storage, the external storage being connected with one of the database processing devices and each of the database processing devices having a buffer memory, a scan processing being performed on the buffer memory to which a predetermined unit of data is read from the database, comprising the steps of:

in the one database processing device,

(a) if a scan request for the database is issued, checking whether or not the target data is in the buffer memories of all database processing devices;

(b) if the target data is in the buffer memory of another database processing device, issuing a scan instruction to the another database processing device;

(c) if no target data is in any of the buffer memories of all of the database processing devices;

(c1) checking whether or not any free area having the predetermined unit size is in the buffer memory of the one database processing device;

(c2) if no free area is in the buffer memory of the one database processing device, moving the content of area of its buffer memory to the buffer memory of one of the other database processing devices to make free area; and

(c3) reading the data from the database to the free area in the buffer memory of the one database processing device; and

(d) executing a scan process on the buffer memory storing the target data.

14. A scanning method executed in a database system having a plurality of database processing devices and a communication line for connecting the database processing devices, a database being stored in an external storage, the external storage being connected with one of the database processing devices and each of the database processing device having a buffer memory, a scan processing being performed on the buffer memory to which a predetermined unit data is read from the database, comprising the steps of:

in the one database processing device,

(a) if a scan request for the database is issued, checking whether or not the target data is in the buffer memories of all of the database processing devices;

(b) if the target data is in the buffer memory of another database processing device, transferring the target data from the another database processing device to the buffer memory of the one database processing device;

(c) if no target data is in the buffer memories of all of the database processing devices;

(c1) checking whether or not any free area having the predetermined unit size is in the buffer memory of the one database processing device;

(c2) if no free area is in the buffer memory of the one database processing device, moving the content of area of the buffer memory of the one database processing device to the buffer memory of one of the other of the database processing devices to make a free area; and

(c3) reading the data from the database to the free area in the buffer memory of the one database processing device; and

(d) executing a scan process on the buffer memory storing the target data.