Alan Wood, 50, was bound and tortured in his home in Lound, near Bourne, in October 2009.
Det Supt Stuart Morrison, who has led the investigation from the start, said he thinks about the case every day.
He said the main focus of the inquiry was to find the person whose DNA profile was left at the scene.
Det Supt Morrison said the profile was not in the UK database and officers were continuing to check international databases.
He said he had taken guidance from Professor Sir Alec Jeffreys, the pioneer of DNA profiling, as to whether it would be worth trying to trace which country the person who left the DNA at the murder scene came from.
"Whilst there are companies that will seek to tell you that information, it's not particularly reliable and it covers so many countries that it would be of no particular practical use for us," the detective told BBC Radio Lincolnshire.
Mr Wood, who ran a small gardening business, had no known enemies, according to Lincolnshire Police.
He was subjected to a "brutal and sustained" attack at his home in Lound, during which he had his throat cut a number of times and an attempt was made to decapitate him.
"I have to be optimistic that this person or people will be found. It's an investigation that I turn over in my mind every single day.
"It is something that you want to do [solve the case] for the victim and for the family, so we occupy quite a lot of our time thinking about it," Det Supt Morrison said.
Manor Lodge, where Mr Wood lived, was kept as a crime scene for two years but has recently been demolished.

The 24-year-old sustained the injury to his right knee during last week's EFL Cup fourth-round win over Tottenham.
It is not the same knee in which he suffered an anterior cruciate ligament injury in October 2015 that sidelined him for seven months.
"To say we are gutted would be an understatement," said Liverpool boss Jurgen Klopp.
"He is such a great boy and has worked so hard that he deserves better luck than this.
"It is typical of Danny's personality, on and off the pitch, that he picked up this injury to his knee when courageously trying to win the ball back for his team by making a tackle.
"But it is this same personality that means he will come back as good and as strong from this latest setback as he has done in the past.
"He is mentally strong, physically strong and will have the very best treatment and rehab possible to make sure when he returns he is ready to play for us at the level we know he is capable of."
England international Ings will have surgery in London on Tuesday.
He joined Liverpool from Burnley in June 2015 for a fee that was later set at £8m by a tribunal.
However, injuries have limited him to just 11 appearances for the Reds since then, in which he has scored four times.
He made his one and only England appearance as a substitute in the 3-0 Euro 2016 qualifier in Lithuania in October 2015, shortly before he suffered his anterior cruciate ligament injury.
The detective leading an inquiry into the brutal murder of a Lincolnshire man has said he remains determined to bring those responsible to justice.
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Liverpool striker Danny Ings will be out for up to nine months with a knee injury that requires surgery.