Abstract:
An apparatus, program product and method that employ a dynamic use of Look Ahead Predicate Generation that will enable the database, database engine, and/or query optimizer to alter the processing of a query, and react to sub-optimal access plan selection and additional factors arising after processing has begun, to heal many poor performing queries. Conventional use of LPG requires the query optimizer to decide whether or not to use LPG before the processing of a query begins. As a result, the query optimizer may not only make sub-optimal decisions, but the query optimizer may not consider additional factors that may arise as the query is processing. However, the dynamic use of LPG allows predicates to be built after processing of the query has started and the ability to alter the query&#39;s processing to improve query performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 11/089,469 filed on Mar. 24, 2005 by Brian Robert Muras, the entire disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to database management systems, and in particular, to the optimization of queries with Look Ahead Predicate Generation by database management systems.  
       BACKGROUND OF THE INVENTION  
       [0003]     Databases are used to store information for an innumerable number of applications, including various commercial, industrial, technical, scientific and educational applications. As the reliance on information increases, both the volume of information stored in most databases, as well as the number of users wishing to access that information, likewise increases. Moreover, as the volume of information in a database, and the number of users wishing to access the database, increases, the amount of computing resources required to manage such a database increases as well.  
         [0004]     Database management systems (DBMS&#39;s), which are the computer programs that are used to access the information stored in databases, therefore often require tremendous resources to handle the heavy workloads placed on such systems. As such, significant resources have been devoted to increasing the performance of database management systems with respect to processing searches, or queries, to databases.  
         [0005]     Improvements to both computer hardware and software have improved the capacities of conventional database management systems. For example, in the hardware realm, increases in microprocessor performance, coupled with improved memory management systems, have improved the number of queries that a particular microprocessor can perform in a given unit of time. Furthermore, the use of multiple microprocessors and/or multiple networked computers has further increased the capacities of many database management systems. From a software standpoint, the use of relational databases, which organize information into formally-defined tables consisting of rows and columns, and which are typically accessed using a standardized language such as Structured Query Language (SQL), has substantially improved processing efficiency, as well as substantially simplified the creation, organization, and extension of information within a database.  
         [0006]     Furthermore, significant development efforts have been directed toward query “optimization,” whereby the execution of particular searches, or queries, is optimized in an automated manner to minimize the amount of resources required to execute each query. A query optimizer typically generates, for each submitted query, an access plan. The access plan may include the use of Look Ahead Predicate Generation (LPG). LPG is a technology used in the iSeries DB2 from International Business Machines Corporation whereby local selection is generated on one table by obtaining the values of columns it joins to on other tables. Typically, the optimizer determines whether LPG should be used or not before it starts to fetch rows for the query. Once the optimizer determines that LPG will be used, the predicates are generated or built, and the entire query is typically processed with the predicates. LPG is typically used on clearly complex queries, since the costs (e.g., time and resources) of processing a complex query without predicates are higher than the costs associated with generating the predicates and then processing the complex query using the predicates. The performance of long running complex join queries, for example, is greatly enhanced through the use of LPG. Contrarily, LPG is not used on clearly simple queries, since the costs of processing a simple query without predicates are lower than the costs associated with generating the predicates and processing the simple query using the predicates.  
         [0007]     Typically, however, for the majority of queries received by the optimizer, it is not precisely clear whether or not the query would benefit from the use of LPG. The lack of clarity by the optimizer may be due to files being dynamically updated as the query is simultaneously processing, statistical imprecision during optimization, or contention for system resources. The lack of clarity may lead to poor decision making by the optimizer and a decline in performance as queries that would benefit from LPG, such as a query that becomes complex during processing due to a large dynamic upload, may be processed without LPG. Moreover, the lack of clarity may lead the optimizer to needlessly decide to process a query using LPG even though the query may not benefit from LPG, thus, increasing the query&#39;s processing time instead of shortening the processing time.  
         [0008]     One cause of such poor decision making is due to the fact that the optimizer typically makes its determination of whether or not to use LPG before initiating processing of the query. Therefore, any additional factors that may arise after processing of the query begins are never considered when deciding whether or not LPG should be used.  
         [0009]     A need therefore exists in the art for improving the performance of database queries, and in particular, for a more flexible and intelligent approach for utilizing LPG in connection with processing database queries.  
       SUMMARY OF THE INVENTION  
       [0010]     The invention addresses these and other problems associated with the prior art by providing an apparatus, program product and method that dynamically uses Look Ahead Predicate Generation (dynamic LPG) after a database query has begun processing. Typically, dynamic LPG will enable a query optimizer to alter the processing of a database query after processing has begun to better optimize query performance, e.g., to react to sub-optimal access plans and additional factors affecting the query&#39;s processing.  
         [0011]     For example, certain embodiments consistent with the invention may perform LPG for a database query after the query has begun processing and use the LPG predicates to fetch at least one additional record from the database by the database query. Furthermore, LPG information can be stored, updated, and retrieved from a cached access plan for the database query to enable the query optimizer to more efficiently determine when to use LPG for the database query, resulting in improved query performance.  
         [0012]     These and other advantages and features, which characterize the invention, are set forth in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, and of the advantages and objectives attained through its use, reference should be made to the Drawings, and to the accompanying descriptive matter, in which there is described exemplary embodiments of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a block diagram of a networked computer system incorporating a database management system within which is implemented query optimization consistent with the invention.  
         [0014]      FIG. 2  is a block diagram illustrating the principal components and flow of information therebetween in the database management system of  FIG. 1 .  
         [0015]      FIG. 3  is a flowchart illustrating the program flow of one implementation of dynamic Look Ahead Predicate Generation. 
     
    
     DETAILED DESCRIPTION  
       [0016]     The embodiments discussed hereinafter illustrate the dynamic use of Look Ahead Predicate Generation (dynamic LPG) after a query has begun processing to improve the query&#39;s performance. Embodiments consistent with the invention may have applicability with practically any type of query that may benefit from LPG.  
         [0017]     Dynamic LPG can generate or build at least one predicate at least one time throughout the processing of the database query. Dynamic LPG may also generate one or more than one predicate at the same time. The query optimizer may determine when an additional predicate may be used.  
         [0018]     Dynamic LPG may be implemented in a number of manners consistent with the invention. For example, one or multiple predicates can be generated based upon whether a threshold has expired. A threshold consistent with this invention may be represented using a number of different metrics, e.g., an input/output cost, a comparison of input/output costs, processing cost, a comparison of processing costs, a time frame, a sliding scale value, a system resource, etc. The threshold may be an exact value such as zero, an estimate, and/or a percentage. The threshold may involve a calculation. For example, the calculation may involve comparing the percentage of actual processing time used to an estimated time and determining if the actual processing time used is a threshold percentage worse than the estimated time. The threshold may also be global to the whole database query or the threshold may be isolated to at least one portion of the database query. Additionally, more than one threshold may also exist for the database query. When multiple thresholds exist, the query optimizer may determine which specific predicate or predicates will be generated by which threshold. For example, the query optimizer may prioritize between multiple predicates based on selectivity and/or cost to generate the predicates. Moreover, determining which specific predicates may be built can be decided before or after a threshold expires. Thus, determining which specific predicates to generate and the actual generation of the predicates may be done separately. Furthermore, the determination need not be dependent on the threshold and/or the determination can be made more than once consistent with the invention.  
         [0019]     Dynamic LPG may also generate at least one predicate in parallel with the processing of the database query, i.e., concurrently with at least one record being fetched from a database by the database query. Thus, dynamic LPG can generate predicates as the database query is being simultaneously processed without predicates. Embodiments consistent with this invention may also use dynamic LPG to generate at least one predicate at the same point in time the database query begins processing. In the alternative, processing of a query may be temporarily suspended while a predicate is being generated.  
         [0020]     Additionally, embodiments consistent with the invention may also dynamically terminate the generation of at least one predicate. The same or similar considerations used to generate predicates with dynamic LPG may be used to terminate the generation of at least one predicate while the database query is processing in parallel. For example, a threshold can also be used to terminate LPG. Moreover, if the processing of the database query completes before a predicate is generated by dynamic LPG, there may no longer be a need to continue generating the predicate, and dynamic LPG can be terminated.  
         [0021]     Embodiments consistent with this invention may therefore generate and/or terminate generating at least one predicate at any time throughout the database query&#39;s processing. Additionally, embodiments consistent with the invention may also improve optimizer performance by storing, updating, and retrieving LPG information from cached query access plans. LPG information consistent with this invention may be any information useful to dynamically initiate and/or terminate dynamic LPG, e.g., an indication that look ahead predicate generation should be performed, an indication that look ahead predicate generation should not be performed, an indication of whether or not performing look ahead predicate generation for the database query improved the processing time for the database query, a result of the look ahead predicate generation, a predicate, a threshold, etc. Therefore, the optimizer can use the cached access plan of a database query in determining whether to use LPG for the database query.  
         [0022]     Turning now to the Drawings, wherein like numbers denote like parts throughout the several views,  FIG. 1  illustrates an exemplary hardware and software environment for an apparatus  10  suitable for implementing a database management system incorporating query optimization consistent with the invention. For the purposes of the invention, apparatus  10  may represent practically any type of computer, computer system or other programmable electronic device, including a client computer, a server computer, a portable computer, a handheld computer, an embedded controller, etc. Moreover, apparatus  10  may be implemented using one or more networked computers, e.g., in a cluster or other distributed computing system. Apparatus  10  will hereinafter also be referred to as a “computer,” although it should be appreciated that the term “apparatus” may also include other suitable programmable electronic devices consistent with the invention.  
         [0023]     Computer  10  typically includes a central processing unit (CPU)  12  including one or more microprocessors coupled to a memory  14 , which may represent the random access memory (RAM) devices comprising the main storage of computer  10 , as well as any supplemental levels of memory, e.g., cache memories, non-volatile or backup memories (e.g., programmable or flash memories), read-only memories, etc. In addition, memory  14  may be considered to include memory storage physically located elsewhere in computer  10 , e.g., any cache memory in a processor in CPU  12 , as well as any storage capacity used as a virtual memory, e.g., as stored on a mass storage device  16  or on another computer coupled to computer  10 .  
         [0024]     Computer  10  also typically receives a number of inputs and outputs for communicating information externally. For interface with a user or operator, computer  10  typically includes a user interface  18  incorporating one or more user input devices (e.g., a keyboard, a mouse, a trackball, a joystick, a touchpad, and/or a microphone, among others) and a display (e.g., a CRT monitor, an LCD display panel, and/or a speaker, among others). Otherwise, user input may be received via another computer or terminal, e.g., via a client or single-user computer  20  coupled to computer  10  over a network  22 . This latter implementation may be desirable where computer  10  is implemented as a server or other form of multi-user computer. However, it should be appreciated that computer  10  may also be implemented as a standalone workstation, desktop, or other single-user computer in some embodiments.  
         [0025]     For non-volatile storage, computer  10  typically includes one or more mass storage devices  16 , e.g., a floppy or other removable disk drive, a hard disk drive, a direct access storage device (DASD), an optical drive (e.g., a CD drive, a DVD drive, etc.), and/or a tape drive, among others. Furthermore, computer  10  may also include an interface  24  with one or more networks  22  (e.g., a LAN, a WAN, a wireless network, and/or the Internet, among others) to permit the communication of information with other computers and electronic devices. It should be appreciated that computer  10  typically includes suitable analog and/or digital interfaces between CPU  12  and each of components  14 ,  16 ,  18 , and  24  as is well known in the art.  
         [0026]     Computer  10  operates under the control of an operating system  26 , and executes or otherwise relies upon various computer software applications, components, programs, objects, modules, data structures, etc. For example, a database management system (DBMS)  28  may be resident in memory  14  to access a database  30  resident in mass storage  16 . Moreover, various applications, components, programs, objects, modules, etc. may also execute on one or more processors in another computer coupled to computer  10  via a network, e.g., in a distributed or client-server computing environment, whereby the processing required to implement the functions of a computer program may be allocated to multiple computers over a network.  
         [0027]     In general, the routines executed to implement the embodiments of the invention, whether implemented as part of an operating system or a specific application, component, program, object, module or sequence of instructions, or even a subset thereof, will be referred to herein as “computer program code,” or simply “program code.” Program code typically comprises one or more instructions that are resident at various times in various memory and storage devices in a computer, and that, when read and executed by one or more processors in a computer, cause that computer to perform the steps necessary to execute steps or elements embodying the various aspects of the invention. Moreover, while the invention has and hereinafter will be described in the context of fully functioning computers and computer systems, those skilled in the art will appreciate that the various embodiments of the invention are capable of being distributed as a program product in a variety of forms, and that the invention applies equally regardless of the particular type of computer readable signal bearing media used to actually carry out the distribution. Examples of computer readable signal bearing media include but are not limited to recordable type media such as volatile and non-volatile memory devices, floppy and other removable disks, hard disk drives, magnetic tape, optical disks (e.g., CD-ROMs, DVDs, etc.), among others, and transmission type media such as digital and analog communication links.  
         [0028]     In addition, various program code described hereinafter may be identified based upon the application within which it is implemented in a specific embodiment of the invention. However, it should be appreciated that any particular program nomenclature that follows is used merely for convenience, and thus the invention should not be limited to use solely in any specific application identified and/or implied by such nomenclature. Furthermore, given the typically endless number of manners in which computer programs may be organized into routines, procedures, methods, modules, objects, and the like, as well as the various manners in which program functionality may be allocated among various software layers that are resident within a typical computer (e.g., operating systems, libraries, API&#39;s, applications, applets, etc.), it should be appreciated that the invention is not limited to the specific organization and allocation of program functionality described herein.  
         [0029]     Those skilled in the art will recognize that the exemplary environment illustrated in  FIG. 1  is not intended to limit the present invention. Indeed, those skilled in the art will recognize that other alternative hardware and/or software environments may be used without departing from the scope of the invention.  
         [0030]      FIG. 2  next illustrates in greater detail the principal components in one implementation of DBMS  28 . The principal components of DBMS  28  that are generally relevant to query execution are a Structured Query Language (SQL) parser  40 , query optimizer  42  and database engine  44 . SQL parser  40  receives from a user (or more typically, an application executed by that user) a database query  46 , which in the illustrated embodiment, is provided in the form of an SQL statement. SQL parser  40  then generates a parsed statement  48  therefrom, which is passed to optimizer  42  for query optimization. As a result of query optimization, an execution or access plan  50  is generated. Once generated, the execution plan is forwarded to database engine  44  for execution of the database query on the information in database  30 . The result of the execution of the database query is typically stored in a result set, as represented at block  52 .  
         [0031]     To facilitate the optimization of queries, DBMS  28  may also include a statistics manager  54 . Statistics manager  54  may be used to gather, create, and analyze statistical information used by the query optimizer  42  to select an access plan. The access plan may or may not include LPG. However, the query optimizer  42  may also create a database query access plan that contains the knowledge that LPG may be dynamically performed at least once throughout the query&#39;s processing. The query optimizer  42  may also store, update, and/or retrieve information from the database query&#39;s cached access plan. Additionally, the database engine  44  may monitor the performance of the database query&#39;s processing and may generate and/or terminate LPG predicates. Database  30  may detect and report changes to any tables in the query to the database engine  44 . It will be appreciated by those of ordinary skill in the art, however, that the optimizer  42 , database  30 , database engine  44  and/or statistics manager  54  may be accorded different functionality to implement dynamic LPG in other embodiments consistent with the invention. Moreover, components may be added and/or omitted in other embodiments consistent with the invention. Those of ordinary skill in the art will also recognize that the exemplary implementation of DBMS  28  illustrated in  FIG. 2  is not intended to limit the present invention. Indeed, those skilled in the art will recognize that other alternative hardware and/or software environments may be used without departing from the scope of the invention.  
         [0032]      FIG. 3  illustrates a flowchart of an exemplary method illustrating the dynamic use of Look Ahead Predicate Generation (LPG) in accordance with the principles of the present invention. In block  60 , the optimizer determines if LPG should be clearly used or not used to process a given query. LPG should be clearly used when running and/or the estimated cost of running the database query with LPG is much lower than the estimated cost of running the database query without LPG and the statistical confidence for the estimate is very high. LPG should be clearly not used when running and/or the estimated cost of running the database query with LPG is much higher than the estimated cost of running the database query without LPG and the statistical confidence for the estimate is very high. The query optimizer may also determine whether LPG should be clearly used or not by retrieving information from the database query&#39;s cached access plan about prior run times. If block  60  determines that LPG clearly should or should not be used, control passes to block  61  to process the query with or without LPG as appropriate, completing this exemplary method. However, if it is unclear whether LPG should be clearly used or not, then control passes to block  62 , which initiates both a NON-LPG and a LPG task for the database query. The NON-LPG and LPG tasks run in parallel and respectively pass control to blocks  64  and  74 . The parallel tasks allows the processing of a database query to begin without LPG and for predicates to be dynamically generated at least once during the query&#39;s processing.  
         [0033]     Turning to block  74 , the LPG task may dynamically start building at least one predicate when a threshold has expired or been met. A threshold consistent with this invention may be represented using a number of different metrics or indicators not limited to those referenced above. The threshold may be continuously checked until the database query has finished processing. If the threshold has expired, then control passes to block  76  to build LPG predicates. Block  76  may build at least one predicate. The specific predicate or predicates to be built by block  76  may be determined before the threshold in block  74  expires as well as after the threshold expires. Next, the LPG predicate or predicates are added to the fetching query in block  78 . The fetching query is the database query which may have been fetching records from the database in the parallel NON-LPG task without any predicates. Embodiments consistent with this invention need not add all the predicates built in block  76  to the fetching query at the same time. After at least one predicate is added, the query can then fetch additional records using the predicates to improve the performance of the query. Those of ordinary skill in the art will appreciate that the threshold and/or predicates can also be stored and/or updated in the query&#39;s cached access plan.  
         [0034]     Turning to block  64 , a record may be fetched without LPG predicates according to the parallel NON-LPG task. Then, in block  66 , it is determined if the query has finished fetching all the records meeting the query parameters. If more records need to be fetched by the query, then control returns to block  64  to fetch more records without the use of predicates. However, blocks  64  and  66  may also be used to fetch records using LPG predicates. For example, if LPG predicates were added to the query by the parallel LPG task in block  78 , then the query may be fetching and determining records meeting the query parameters in blocks  64  and  66  using at least one LPG predicate.  
         [0035]     Nonetheless, when the query is done fetching, it is determined in block  68  if predicates are still being built by the parallel LPG task. Given that the NON-LPG and LPG tasks are running in parallel, determining if LPG is still building in block  68  refers to predicates being generated by the parallel LPG task. If LPG is still building, then control passes to block  72  which terminates the building of any predicates by the parallel LPG task. One of ordinary skill in the art will recognize that if the query has already finished fetching all the records meeting the query parameters in block  66 , then the need may no longer exist to continue building predicates to process a query that has already finished. Therefore, unless the LPG task has ended, the LPG task may be terminated in block  72 . However, embodiments consistent with this invention may not terminate the LPG task. Instead, for example, an embodiment may continue building predicates already in progress and/or may store and/or update any information (e.g., threshold, partially generated predicates, predicates that have not been added to the fetching query, etc.) relevant to the LPG task in the cached access plan for the database query prior to terminating the LPG task in block  72 . Those of ordinary skill in the art will appreciate that although the query may have finished processing before the LPG task completed, this approach may be useful to future queries and also lead to improved query performance and improved decision making by the query optimizer.  
         [0036]     Returning to block  68 , if the LPG task is not building predicates, then control passes to block  70 . If the LPG task is not building, this may mean that the LPG task ended and the predicates were already added to the query. Block  70  updates the cached access plan for the database query with whether or not LPG improved the processing of the query. Those of ordinary skill in the art will appreciate the benefits of updating the query&#39;s cached access plan, for example, if the same query needs to be processed, the optimizer can read the cached access plan and clearly determine if LPG should be used before processing begins in block  60 . Additionally, other information (e.g., a result of the look ahead predicate generation, a predicate, a threshold, etc.) may also be stored and/or updated in the cached access plan for the database query consistent with the invention.  
         [0037]     The following example demonstrates the advantages of the illustrated embodiments over conventional LPG methodologies. In this example, the query optimizer may receive a query such as: 
        Q 1 : Select*from 
            Fact, D 1 , D 2     
            where 
            Fact.D 1 ID=D 1 .ID and Fact.D 2 ID=D 2 .ID and     D 1 .flag=y and D 2 .status= 200     
               
 
         [0043]     Upon receiving this query, the optimizer may not be precisely clear as to whether LPG should be used before processing of the query commences. The optimizer may determine that the cost of generating the predicates and processing Q 1  with the predicates is higher than the costs of processing Q 1  without the predicates. Thus, the optimizer may commence processing of Q 1  without LPG by joining in a left to right manner and probing the Fact table as shown in TABLE I, the D 1  table as shown in TABLE II, and the D 2  table as shown in TABLE III.  
                                           TABLE I                           FACT TABLE                D1ID   D2ID                            1   a           1   b           2   c           2   d           3   e           3   f           4   g           4   h           5   i           5   j           6   a           6   b           7   c           7   d           8   e           8   f           9   g           9   h           10   i           10   j                      
 
         [0044]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                   
               
               
                 D1 TABLE 
               
             
          
           
               
                   
                 ID 
                 flag 
               
               
                   
                   
               
             
          
           
               
                   
                 1 
                 y 
               
               
                   
                 2 
                 y 
               
               
                   
                 3 
                 y 
               
               
                   
                 4 
                 y 
               
               
                   
                 5 
                 y 
               
               
                   
                 6 
                 n 
               
               
                   
                 7 
                 n 
               
               
                   
                 8 
                 n 
               
               
                   
                 9 
                 n 
               
               
                   
                 10 
                 n 
               
               
                   
                   
               
             
          
         
       
     
         [0045]    
       
         
               
             
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                   
               
               
                 D2 TABLE 
               
             
          
           
               
                   
                 ID 
                 status 
               
               
                   
                   
               
               
                   
                 a 
                 100 
               
               
                   
                 b 
                 100 
               
               
                   
                 c 
                 100 
               
               
                   
                 d 
                 200 
               
               
                   
                 e 
                 200 
               
               
                   
                 f 
                 200 
               
               
                   
                 g 
                 300 
               
               
                   
                 h 
                 300 
               
               
                   
                 i 
                 300 
               
               
                   
                 j 
                 500 
               
               
                   
                   
               
             
          
         
       
     
         [0046]     By not using LPG, there may be twenty sequential probes into the Fact table and twenty probes into the D 1  table, with ten probes finding a match on D 1 &#39;s selection. Furthermore, the ten values may be used to probe the D 2  table. Thus, without LPG, fifty probes may be required to return the three records that met the parameters of Q 1  as shown in TABLE IV.  
                             TABLE IV                           RESULT                Fact.D1ID   Fact.D2ID                       2   d           3   e           3   f                      
 
         [0047]     However, instead of incurring the cost of the fifty probes, LPG predicates may be dynamically generated after processing of Q 1  has begun with the illustrated embodiment to reduce the number of probes. Moreover, if the optimizer was not certain if LPG should be clearly used or not before processing a database query, or additional factors relevant the processing of the query arose after processing began, the LPG task may dynamically build predicates and add the predicates to the query. Using the same example, eighteen total probes may have been used to process records ( 1 , a) through ( 3 , f) inclusive from the Fact table by the NON-LPG task. The threshold may have expired due to the eighteen probes. As a result, at least one predicate, Fact.D 2 ID in (‘d’,‘e’,‘f’), may be built and added to Q 1 . While the predicate is building, the NON-LPG task may continue to process the next record, ( 4 , g), from the Fact table. Once the predicate is done building, it may be added to Q 1 . Q 1  may be internally rewritten as: 
        Q 1 : Select*from 
            Fact, Dimension 1 , Dimension 2     
            where 
            Fact.D 1 ID=D 1 .ID and Fact.D 2 ID=D 2 .ID and     D 1 .flag=y and D 2 .status= 200      and Fact.D 2 ID in (‘d’,‘e’,‘f’) 
 
 After the predicate has been added to Q 1 , starting with record ( 4 , h), Q 1  may process much faster from that point onwards since most of the remaining records in the Fact table may be discarded if they are not in (‘d’,‘e’,‘f’). If Q 1 , which now includes one predicate, is still not processing fast enough according to some threshold, the LPG task may build another predicate, Fact.D 1 ID in ( 1 , 2 , 3 , 4 , 5 ), and add it to Q 1 . Q 1  may be internally rewritten as: 
   
            Q 1 : Select*from 
            Fact, Dimension 1 , Dimension 2     
            where 
            Fact.D 1 ID=D 1 .ID and Fact.D 2 ID=D 2 .ID and     D 1 .flag=y and D 2 .status= 200      and Fact.D 2 ID in (‘d’,‘e’,‘f’)     and Fact.D 1 ID in ( 1 , 2 , 3 , 4 , 5 ) 
 
 The second predicate will additionally enhance the performance of Q 1  by discarding additional records not in ( 1 , 2 , 3 , 4 , 5 ). Where the conventional approach required the choice of whether to use LPG or not before processing of the query began, dynamic LPG provides a more flexible and intelligent approach such that the number of probes by conventional techniques, fifty probes, may be reduced after processing of the database query has begun. Therefore, the dynamic use of LPG allows the processing of Q 1  to be altered in light of sub-optimal decision-making and additional factors arising after processing of the query has begun, resulting in a reduced number of probes and improved query performance. 
   
               
 
         [0061]     Various additional modifications may be made to the illustrated embodiments without departing from the spirit and scope of the invention. Therefore, the invention lies in the claims hereinafter appended.