I am always missing a Custom Field that can handle Key=Value entried; combined with multiple entries, so I use a multiline Text Field:
Service Level=AppDrift CustomerNumber=1043 Othername=Myhost
Lets call the field "VC Tags". Issues with a Key=Value can be found with a JQL like:
"VC Tags"~"Service Level=AppDrift"
But is quite volatile and the fuzzy search ~ will also find:
Service Level=AppDrift+ CustomerNumber=1043 Othername=Myhost
So, this JQL Function (For Adaptavist ScriptRunner) can cope with that:
This is working sample code, there is room for better Error Handling 
package com.onresolve.jira.groovy.jql
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.Issue
import com.atlassian.query.clause.TerminalClause
import com.atlassian.jira.jql.query.QueryCreationContext
import org.apache.lucene.index.Term
import com.onresolve.jira.groovy.jql.AbstractScriptedJqlFunction
import org.apache.lucene.search.BooleanClause
import org.apache.lucene.search.BooleanQuery
import org.apache.lucene.search.Query
import org.apache.lucene.search.TermQuery
import com.atlassian.jira.issue.fields.CustomField
import com.atlassian.jira.issue.CustomFieldManager
import com.atlassian.jira.user.ApplicationUser
import com.atlassian.jira.security.JiraAuthenticationContext
import com.atlassian.query.operand.FunctionOperand
import com.atlassian.jira.issue.Issue
class keyValueInField extends AbstractScriptedJqlFunction implements JqlQueryFunction {
@Override
String getDescription() {
"Function to find a key value pair in a Custom Field"
}
@Override
List<Map> getArguments() {
[
[ "subquery": "Subquery", "optional": false],
[ "fieldname": "Custom Fieldname", "optional": false],
[ "key": "", "The Key to find": false],
[ "value": "", "The value to check for in the key": false],
[ "usecontains": "", "Use contains instead of equal": false]
]
}
@Override
String getFunctionName() {
"keyValueInField"
}
@Override
Query getQuery(QueryCreationContext queryCreationContext, FunctionOperand operand, TerminalClause terminalClause) {
CustomFieldManager customFieldManager = ComponentAccessor.getCustomFieldManager()
final JiraAuthenticationContext context = ComponentAccessor.getJiraAuthenticationContext()
final ApplicationUser applicationUser = context.getLoggedInUser()
final BooleanQuery.Builder boolQueryBuilder = new BooleanQuery.Builder()
String jql = operand.args[0]
String fieldname = operand.args[1]
String key = operand.args[2].toLowerCase()
String value = operand.args[3].toLowerCase()
String usecontains = operand.args[4].toLowerCase()
issues = getIssues(jql, applicationUser)
issues.each { Issue issue ->
//Get Field
def field = customFieldManager.getCustomFieldObjectsByName(fieldname)[0]
if (field)
{
//Get the Value
def fieldValue = issue.getCustomFieldValue(field)?:""
if (fieldValue)
{
//Loop each Value
fieldValue.toString().split("\n").each { theLine ->
theLine = theLine.toLowerCase()
if (usecontains == "1" || usecontains == "true")
{
if (theLine.split("=")[0] == key && theLine.split("=")[1].contains(value))
{
boolQueryBuilder.add(new TermQuery(new Term("issue_id", issue.id as String)), BooleanClause.Occur.SHOULD)
}
}
else
{
if (theLine.split("=")[0] == key && theLine.split("=")[1] == value)
{
boolQueryBuilder.add(new TermQuery(new Term("issue_id", issue.id as String)), BooleanClause.Occur.SHOULD)
}
}
}
}
}
}
return boolQueryBuilder.build()
}
}
Usage:
Issuefunction in keyValueInField("Project=TEST","VC Tags","Service Level","AppDrift",0)
for an equal search (0). For a contains (fuzzy ~) search:
ssuefunction in keyValueInField("Project=TEST and IssueType=Task","VC Tags","Service Level","AppDrift",1)