SOQL queries must be selective, particularly for queries inside triggers for the best performance. To avoid long execution times, non-selective SOQL queries may be terminated by the system.

Selective SOQL Query Criteria

A query is selective when one of the query filters is on an indexed field and the query filter reduces the resulting number of rows below a system-defined threshold. The performance of the SOQL query improves when two or more filters used in the WHERE clause meet the mentioned conditions.

Custom Index Considerations for Selective SOQL Queries

The following fields are indexed by default:

• Primary keys (ID, Name, and Owner fields).
• Foreign keys (lookup or master-detail relationship fields).
• Audit dates (such as SystemModStamp). 
• Custom fields marked as External ID or Unique.

A Custom Index can’t be created on these types of fields:

• Multi-select Picklists.
• Currency fields in a Multicurrency Organization.
• Long text fields. 
• Binary fields (fields of type blob, file, or encrypted text). 

Examples of Selective SOQL Queries   

To better understand whether a query on a large object is selective or not, let’s analyze some queries. For these queries, we will assume there are more than 200,000 records (including soft-deleted records, that is, deleted records that are still in the Recycle Bin) for the Account sObject.

Query 1:

SELECT Id FROM Account WHERE Id IN (<list of account IDs>)

The WHERE clause is on an indexed field (Id). If SELECT COUNT() FROM Account WHERE Id IN (<list of account IDs>) returns fewer records than the selectivity threshold, the index on Id is used. This will typically be the case since the list of IDs only contains a small amount of records.

Query 2:

SELECT Id FROM Account WHERE Name != ''

Since Account is a large object even though Name is indexed (primary key), this filter returns most of the records, making the query non-selective.

Query 3:

SELECT Id FROM Account WHERE Name != '' AND CustomField__c = 'ValueA'

Here we have to see if each filter, when considered individually, is selective. As we saw in the previous example the first filter isn’t selective. So let’s focus on the second one. If the count of records returned by SELECT COUNT() FROM Account WHERE CustomField__c = ‘ValueA’ is lower than the selectivity threshold, and CustomField__c is indexed, the query is selective.

Query 4:

SELECT Id FROM Account WHERE FormulaField__c = 'ValueA'

The following rules have to be true in order to index a Formula Field:

• The formula contains fields from a single object only (not relationship fields).
• The formula field doesn’t reference any non-deterministic functions (e.g. SYSDATE).
• The formula field doesn’t reference any non-supported fields for including in indexes.  This list isn’t documented anywhere specifically (there are lots of special cases), but in Spring 12(176), createdById was non-supported, but in Summer 12 (178), it is supported.  Same story for CreatedDate.
• The formula field doesn’t contain references to Primary Keys (e.g Id) 
• The formula field does not use TEXT(<picklist-field>) function
• If the formula references any Lookup fields, the field must not have the option “What to do if the lookup record is deleted?” set to “Clear the value of this field.”

Enable and Use the Query Plan Tool

1. In the Developer Console, click Help | Preferences
2. Set ‘Enable Query Plan’ to TRUE.

Once enabled in the Developer Console, you can access the Query Plan Tool in the ‘Query Editor‘ tab of the console.

To use the Query Plan Tool, simply enter a query in the Query Editor and press Query Plan to displays all query operations and the cost of each for the SOQL provided.

Reasons to use the Query Plan Tool

Use this tool to check the Query Plan for any SOQL queries that execute slowly. It will provide you with insight on the different plans and should you have some of the filters indexed, provide the cost of using the index compared to a full table scan.

If the cost for the table scan is lower than the index, and the query is timing out, you will need to perform further analysis on using other filters to improve selectivity, or, if you have another selective filter in that query that is not indexed but is a candidate for one.
 

Determine if a filter is selective:

• Determine if it has an index.
• If the filter is on a standard field, it’ll have an index if it is a primary key (Id, Name, OwnerId), a foreign key (CreatedById, LastModifiedById, lookup, master-detail relationship), and an audit field (CreatedDate, SystemModstamp).
• Custom fields will have an index if they have been marked as Unique or External Id
• If the filter doesn’t have an index, it won’t be considered for optimization.
• If the filter has an index, determine how many records it would return:
• For a standard index, the threshold is 30 percent of the first million targeted records and 15 percent of all records after that first million. In addition, the selectivity threshold for a standard index maxes out at 1   million total targeted records, which you could reach only if you had more than 5.6 million total records.
• For a custom index, the selectivity threshold is 10 percent of the first million targeted records and 5 percent all records after that first million.  In addition, the selectivity threshold for a custom index maxes out at 333,333 targeted records, which you could reach only if you had more than 5.6 million records.
• If the filter exceeds the threshold,it won’t be considered for optimization.
• If the filter doesn’t exceed the threshold, this filter IS selective, and the query optimizer will consider it for optimization.

Indexed field does not appear in the list of plans

If the query you provided contains an Indexed field in the filters, the plan will be shown for that field only if you are using a supported operation against that field.

Here is a list of unsupported operations:

• Custom index will never be used when comparisons are being done with an operator like “NOT EQUAL TO”
• Custom index will never be used when comparisons are being done with a null value like “Name = ””
• Leading ‘%’ wildcards are inefficient operators that also make filter conditions non-selective
• When using an OR comparison, all filters must be indexed and under the 10% threshold. . If you have a non-indexed field or one is above 10%, the plan will not be displayed.

Does the Query Plan Tool display index candidates or fields that can be indexed?

The Query Plan tool will only show Indexed field statistics in the Plan’s section, not fields that could be indexed. This tool will not provide information on fields that can be indexed.