fix(accountRecord): Rollback accountRecord_4 due to unexplained performance issues.

Author: rfk

lib/db/mysql.js

@@ -896,9 +896,20 @@ module.exports = function (log, error) {
   // Fields : uid, email, normalizedEmail, emailVerified, emailCode, kA, wrapWrapKb, verifierVersion, authSalt,
   //          verifierSetAt, createdAt, lockedAt, primaryEmail, profileChangedAt
   // Where  : emails.normalizedEmail = LOWER($1)
-  var GET_ACCOUNT_RECORD = 'CALL accountRecord_4(?)'
+  //
+  // There's a newer version of this query named `accountRecord_4`
+  // which pulls the `profileChangedAt` column from the database.
+  // We're experiencing unexpectedly bad query performance for
+  // reasons that we don't yet understand, so we've reverted to
+  // using an older version of the query while we're figuring that out.
+  // Ref: https://github.com/mozilla/fxa-content-server/issues/6655
+  var GET_ACCOUNT_RECORD = 'CALL accountRecord_2(?)'
   MySql.prototype.accountRecord = function (email) {
     return this.readFirstResult(GET_ACCOUNT_RECORD, [email])
+      .then(record => {
+        record.profileChangedAt = record.verifierSetAt || record.createdAt
+        return record
+      })
   }
 
   // Select : emails