Step 1: Scan table t1 and sort using a temporary range index.

Step 2: Scan table t2 and sort using a temporary range index. After scan, return the rows that match: t2.b<>0.

Step 1 and Step 2 are on Level 3.

Step 3: Merge results from Steps 1 and 2 and join the rows that match: t1.a=-t2.a.

Step 4: Scan table t3 and sort using a temporary range index.

Step 3 and Step 4 are on Level 2.

Step 5: Merge results from Steps 3 and 4 and join the rows that match: t2.a=T3.a. After join, return the rows that match: t3.b/t1.b>1.

Step 5 is on level 1.

End of description.