Governor execution limits ensure the efficient use of resources on the Force.com multitenant platform. It is the limit specified by the Salesforce.com on code execution for efficient processing.

What are Governor Limits?

As we know, Apex runs in multi-tenant environment, i.e., a single resource is shared by all the customers and organizations. So, it is necessary to make sure that no one monopolizes the resources and hence Salesforce.com has created the set of limits which governs and limits the code execution. Whenever any of the governor limits are crossed, it will throw error and will halt the execution of program.
From a Developer’s perspective, it is important to ensure that our code should be scalable and should not hit the limits.
All these limits are applied on per transaction basis. A single trigger execution is one transaction.
As we have seen, the trigger design pattern helps avoid the limit error. We will now see other important limits.

Avoiding SOQL Query Limit

You can issue only 100 queries per transaction, that is, when your code will issue more than 100 SOQL queries then it will throw error.
Example
This example shows how SOQL query limit can be reached −
The following trigger iterates over a list of customers and updates the child record’s (Invoice) description with string ‘Ok to Pay’.

// Helper class:Below code needs o be checked.
public class CustomerTriggerHelper {

public static void isAfterUpdateCall(Trigger.new) {
createInvoiceRecords(trigger.new);//Method call
updateCustomerDescription(trigger.new);
}

// Method To Create Invoice Records
public static void createInvoiceRecords (List<apex_customer__c> customerList) {
for (APEX_Customer__c objCustomer: customerList) {

if (objCustomer.APEX_Customer_Status__c == 'Active' &&
trigger.oldMap.get(objCustomer.id).APEX_Customer_Status__c == 'Inactive') {

// condition to check the old value and new value
APEX_Invoice__c objInvoice = new APEX_Invoice__c();
objInvoice.APEX_Status__c = 'Pending';
InvoiceList.add(objInvoice);
}
}
insert InvoiceList; // DML to insert the Invoice List in SFDC
}

// Method to update the invoice records
public static updateCustomerDescription (List<apex_customer__c> customerList) {
for (APEX_Customer__c objCust: customerList) {
List<apex_customer__c> invList = [SELECT Id, Name,
APEX_Description__c FROM APEX_Invoice__c WHERE APEX_Customer__c = :objCust.id];

// This query will fire for the number of records customer list has and will
// hit the governor limit when records are more than 100
for (APEX_Invoice__c objInv: invList) {
objInv.APEX_Description__c = 'OK To Pay';
update objInv;
// Update invoice, this will also hit the governor limit for DML if large
// number(150) of records are there
}
}
}
}

When the ‘updateCustomerDescription’ method is called and the number of customer records are more than 100, then it will hit the SOQL limit. To avoid this, never write the SOQL query in the For Loop. In this case, the SOQL query has been written in the For loop.
Following is an example which will show how to avoid the DML as well as the SOQL limit. We have used the nested relationship query to fetch the invoice records and used the context variable trigger.newMap to get the map of id and Customer records.

// SOQL-Good Way to Write Query and avoid limit exception
// Helper Class
public class CustomerTriggerHelper {
public static void isAfterUpdateCall(Trigger.new) {
createInvoiceRecords(trigger.new); //Method call
updateCustomerDescription(trigger.new, trigger.newMap);
}

// Method To Create Invoice Records
public static void createInvoiceRecords (List<apex_customer__c> customerList) {
for (APEX_Customer__c objCustomer: customerList) {

if (objCustomer.APEX_Customer_Status__c == 'Active' &&
trigger.oldMap.get(objCustomer.id).APEX_Customer_Status__c == 'Inactive') {

// condition to check the old value and new value
APEX_Invoice__c objInvoice = new APEX_Invoice__c();
objInvoice.APEX_Status__c = 'Pending';
InvoiceList.add(objInvoice);
}
}
insert InvoiceList; // DML to insert the Invoice List in SFDC
}

// Method to update the invoice records
public static updateCustomerDescription (List<apex_customer__c>
customerList, Map<id, apex_customer__c> newMapVariable) {
List<apex_customer__c> customerListWithInvoice = [SELECT id,
Name,(SELECT Id, Name, APEX_Description__c FROM APEX_Invoice__r) FROM
APEX_Customer__c WHERE Id IN :newMapVariable.keySet()];

// Query will be for only one time and fetches all the records
List<apex_invoice__c> invoiceToUpdate = new
List<apex_invoice__c>();

for (APEX_Customer__c objCust: customerList) {
for (APEX_Invoice__c objInv: invList) {
objInv.APEX_Description__c = 'OK To Pay';
invoiceToUpdate.add(objInv);
// Add the modified records to List
}
}
update invoiceToUpdate;
}
}

DML Bulk Calls

This example shows the Bulk trigger along with the trigger helper class pattern. You must save the helper class first and then save the trigger.
Note − Paste the below code in ‘CustomerTriggerHelper’ class which we have created earlier.

// Helper Class
public class CustomerTriggerHelper {
public static void isAfterUpdateCall(List<apex_customer__c> customerList,
Map<id, apex_customer__c> mapIdToCustomers, Map<id, apex_customer__c>
mapOldItToCustomers) {
createInvoiceRecords(customerList, mapOldItToCustomers); //Method call
updateCustomerDescription(customerList,mapIdToCustomers,
mapOldItToCustomers);
}

// Method To Create Invoice Records
public static void createInvoiceRecords (List<apex_customer__c>
customerList, Map<id, apex_customer__c> mapOldItToCustomers) {
List<apex_invoice__c> InvoiceList = new List<apex_invoice__c>();
List<apex_customer__c> customerToInvoice = [SELECT id, Name FROM
APEX_Customer__c LIMIT 1];

for (APEX_Customer__c objCustomer: customerList) {
if (objCustomer.APEX_Customer_Status__c == 'Active' &&
mapOldItToCustomers.get(objCustomer.id).APEX_Customer_Status__c == 'Inactive') {
//condition to check the old value and new value
APEX_Invoice__c objInvoice = new APEX_Invoice__c();
objInvoice.APEX_Status__c = 'Pending';
objInvoice.APEX_Customer__c = objCustomer.id;
InvoiceList.add(objInvoice);
}
}
system.debug('InvoiceList&&&'+InvoiceList);
insert InvoiceList;
// DML to insert the Invoice List in SFDC. This also follows the Bulk pattern
}

// Method to update the invoice records
public static void updateCustomerDescription (List<apex_customer__c>
customerList, Map<id, apex_customer__c> newMapVariable, Map<id,
apex_customer__c> oldCustomerMap) {
List<apex_customer__c> customerListWithInvoice = [SELECT id,
Name,(SELECT Id, Name, APEX_Description__c FROM Invoices__r) FROM
APEX_Customer__c WHERE Id IN :newMapVariable.keySet()];

// Query will be for only one time and fetches all the records
List<apex_invoice__c> invoiceToUpdate = new List<apex_invoice__c>();
List<apex_invoice__c> invoiceFetched = new List<apex_invoice__c>();
invoiceFetched = customerListWithInvoice[0].Invoices__r;
system.debug('invoiceFetched'+invoiceFetched);
system.debug('customerListWithInvoice****'+customerListWithInvoice);

for (APEX_Customer__c objCust: customerList) {
system.debug('objCust.Invoices__r'+objCust.Invoices__r);
if (objCust.APEX_Active__c == true &&
oldCustomerMap.get(objCust.id).APEX_Active__c == false) {
for (APEX_Invoice__c objInv: invoiceFetched) {
system.debug('I am in For Loop'+objInv);
objInv.APEX_Description__c = 'OK To Pay';
invoiceToUpdate.add(objInv);
// Add the modified records to List
}
}
}
system.debug('Value of List ***'+invoiceToUpdate);
update invoiceToUpdate;
// This statement is Bulk DML which performs the DML on List and avoids
// the DML Governor limit
}
}

// Trigger Code for this class: Paste this code in 'Customer_After_Insert'
// trigger on Customer Object
trigger Customer_After_Insert on APEX_Customer__c (after update) {
CustomerTriggerHelper.isAfterUpdateCall(Trigger.new, trigger.newMap,
trigger.oldMap);
// Trigger calls the helper class and does not have any code in Trigger
}

Other Salesforce Governor Limits

Following table lists down the important governor limits.

Description	Limit
Total heap size	6 MB/12 MB
Total number of DML statements issued	150
Total number of records retrieved by a single SOSL query	2000
Total number of SOSL queries issued	20
Total number of records retrieved by Database.getQueryLocator	10000
Total number of records retrieved by SOQL queries	50000

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