Why mysql multiple column index match from left? Why MySQL using union index from left? For example union index(a,b,c),(a) could using index,(a,b) could using index,but (b,c) could not using index!!!Any useful tool to probe MySQL index file structure? How could this happen?
it simple and very clear for understanding without technical details:

1,4,1
1,5,1
2,1,4
2,3,1
3,0,2


1st column is sorted

2nd column sorted independent for each cluster in 1st column

3rd column sorted for each 1+2 cluster

so, (2,1,x) will always after (1,4,x), but it is not true for (x,4,1) vs (x,1,4)