The Tueller Drill - or 21 foot rule - shows that a threat with a knife can get in a fatal blow on a person with a holstered handgun if that person is within 21 feet of the threat.
Some people discount the severity of a threat armed with a knife, as did these Nicaraguan police officers. ***Warning - graphic content. Two police officers are killed and several wounded in this video.***
Watching that video showed me how fast one guy with a knife can end lives and maim others. There was quite a bit of shooting from the officers with rifles, and it's unknown how many of their shots connected. The assailant finally did go down, but was still drawing breath when they put the cuffs on him.
I noticed that none of the cops had their weapons trained on the guy when he made his move. There was some complacency there that you can see in this longer version of the video that starts minutes before ol' boy goes all stabby. One well placed shot could have stopped the attack before it began, but once it commenced, the cops were on the defense and the assailant was moving fast. I also noticed that every cop he attacked was trying to retreat and fell down in the process, which is why gun schools often teach their students on proper footwork when moving. I would imagine that this wouldn't have played out like it did if it happened in the US. After he failed to drop the knife in a moment or two, he would have been Tazed, maced, and then savagely beaten before being handcuffed, and probably one officer would have been positioned for a lethal shot from a firearm.