AL Database Methods and Performance on SQL Server

Articolo
05/19/2022

This topic describes the relationship between basic database methods in AL and SQL statements.

Get, Find, FindSet, and Next

The AL language offers several methods to retrieve record data. In Dynamics 365 Business Central, records are retrieved using multiple active result sets (MARS). Generally, retrieving records with MARS is faster than with server-side cursors. Additionally, each function is optimized for a specific purpose. To achieve optimal performance you must use the method that is best suited for a given purpose.

Record.Get is optimized for getting a single record based on primary key values.
Record.Find is optimized for getting a single record based on the primary keys in the record and any filter or range that has been set.
Record.Find('-') and Record.Find('+') are optimized for reading primarily from a single table when the application might not read all records. Find('-') is implemented by issuing a self-tuning TOP X call, where X can change over time, based on statistics of the number of rows read.

The following are examples of scenarios in which you should use the Find('-') function to achieve optimal performance:
- Before you post a general journal batch, you must check all journal lines for validity and verify that all lines balance. After the first line when an error is found, you do not have to retrieve the rest of the rows.
- if you want to fulfill multiple outstanding orders from a recent purchase but you do not know how many orders are covered by the purchase.
Record.FindSet(ForUpdate, UpdateKey) is optimized for reading the complete set of records in the specified filter and range. The UpdateKey parameter does not influence the efficiency of this method in Dynamics 365 Business Central, such as it did in Microsoft Dynamics NAV 2009.

FindSet is not implemented by issuing a TOP X call.
Record.FindFirst and Record.FindLast are optimized for finding the single first or last record in the specified filter and range.
Record.Next can be called at any time. However, if Record.Next is not called as part of retrieving a continuous result set, then Business Central calls a separate SQL statement in order to Find the Next record.

Dynamic result sets

Any result set that is returned from a call to the Find methods discussed in the previous section is dynamic. That means that the result set is guaranteed to contain any changes that you make further ahead in the result set. However, this feature comes at a cost. If any modifications are made to a table which is being traversed, then Business Central might have to issue an extra SQL statement to guarantee that the result set is dynamic.

The following code shows how records are most efficiently retrieved. FindSet is the most efficient method to use because this example reads all records.

if FindSet() then  
    repeat  
        // Insert statements to repeat.  
    until Next() = 0;

CalcFields, CalcSums, and Count

Each call to CalcFields, CalcField, CalcSum, or CalcSums methods that calculates a sum requires a separate SQL statement unless the client has calculated the same sum or another sum that uses the same SumIndexFields or filters in a recent operation, and therefore, the result is cached.

Each CalcFields or CalcSums request should be confined to use only one SIFT index. The SIFT index can only be used if:

All requested sum-fields are contained in the same SIFT index.
The filtered fields are part of the key fields specified in the SIFT index containing all the sum fields.

if neither of these requirements is fulfilled, then the sum will be calculated directly from the base table.

In Dynamics 365 Business Central, SIFT indexes can be used to count records in a filter provided that a SIFT index exists that contains all filtered fields in the key fields that are defined for the SIFT index.

SetAutoCalcFields

It is a common task to retrieve data and request calculation of associated FlowFields. The following example traverses customer records, calculates the balance, and marks the customer as blocked if the customer exceeds the maximum credit limit. Note: the Customer record and associated fields are imaginary in the following examples.

if Customer.FindSet() then 
    repeat  
        Customer.CalcFields(Balance);  
        if Customer.Balance > MaxCreditLimit then begin  
            Customer.Blocked := true;   
            Customer.Modify();  
        end else 
            if Customer.Balance > LargeCredit then begin 
                Customer.Caution := true;  
                Customer.Modify();   
            end;   
    until Customer.Next() = 0;

In Dynamics 365 Business Central, you can do this much faster. First, we set a filter on the customer. This could also be done in Business Central 2009, but behind the scenes the same code as mentioned earlier would be executed. In Dynamics 365 Business Central, setting a filter on a record is translated into a single SQL statement.

Customer.SetFilter(Balance, '>%1' , LargeCredit);   
if Customer.FindSet() then 
    repeat  
        Customer.CalcFields(Balance);  
        if Customer.Balance > MaxCreditLimit then begin   
            Customer.Blocked := true;   
            Customer.Modify();   
        end else 
            if Customer.Balance > LargeCredit then begin   
                Customer.Caution := true;   
                Customer.Modify();   
            end;   
    until Customer.Next() = 0;

In the previous example, an extra call to CalcFields still must be issued for the code to be able to check the value of Customer.Balance. In Dynamics 365 Business Central, you can optimize this further by using the new SetAutoCalcFields method.

Customer.SetFilter(Balance, '>%1', LargeCredit);   
Customer.SetAutoCalcFields(Balance)   
if Customer.FindSet() then 
    repeat   
        if Customer.Balance > MaxCreditLimit then begin   
            Customer.Blocked := true;   
            Customer.Modify();   
        end else 
            if Customer.Balance > LargeCredit then begin   
                Customer.Caution := true;   
                Customer.Modify();   
            end;   
    until Customer.Next() = 0;

Insert, Modify, Delete, and LockTable

Each call to Insert, Modify, or Delete methods requires a separate SQL statement. If the table that you Modify contains SumIndexes, then the operations will be much slower. As a test, select a table that contains SumIndexes and execute one hundred Insert, Modify, or Delete operations to measure how long it takes to maintain the table and all its SumIndexes.

Cloning a record before a Modify or Delete operation issues an extra SQL statement, since the SQL SELECT query is restarted every time the table is cloned. A record will be cloned when calling the Copy Method, when using a RecordRef or when the record is not passed with the var parameter within a function.

The following code samples will lead to a bad performance, since they will issue an extra SQL statement per record in the table.

if MyTable.FindSet() then
    repeat
        MyTableCopy.Copy(MyTable);
        // ...
        MyTableCopy.Modify(); // or .Delete();
    until MyTable.Next() = 0;

if MyTable.FindSet() then
    repeat
        RecRef.GetTable(MyTable);
        // ...
        RecRef.Modify(); // or .Delete();
    until MyTable.Next() = 0;

Instead, you should do the following, which only requires an extra SQL statement:

RecRef.Open(Database::"My Table");
if RecRef.FindSet() then
    repeat
        // ...
        RecRef.Modify(); // or .Delete();
    until RecRef.Next() = 0;

if MyTable.FindSet() then
    repeat
        MyTable.Modify(); // or .Delete();
    until MyTable.Next() = 0;

The LockTable method does not require any separate SQL statements. It will cause any subsequent reading from any tables to be done with an update lock. For more information, see Record.LockTable Method.

ModifyAll and DeleteAll

Using ModifyAll and DeleteAll can improve performance by limiting the amount of SQL calls needed. However, be aware that ModifyAll and DeleteAll will revert to individual calls if any of the following conditions exist:

There is trigger code on the table.
There are event subscribers to the following events: OnBeforeModify, OnAfterModify, OnGlobalModify, OnBeforeDelete, OnAfterDelete, OnGlobalDelete, and OnDatabaseModify.
Security filtering is active.
The table contains Media or MediaSet data type fields.
There are fields that are added through companion tables.

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