WHAT AVG(AMOUNT) MUST READ FROM DISK
AMOUNT = WHAT THE QUERY NEEDS
ROW STORAGE
READS 16 OF 16 VALUES
ROW 1
ROW 2
ROW 3
ROW 4
2041
APAC
128.00
03-11
2042
US
16.40
03-11
2043
EU
73.25
03-12
2044
US
9.10
03-12
READ PATH — THE SCAN TOUCHES EVERY FIELD OF EVERY ROW TO REACH EACH AMOUNT
COLUMNAR STORAGE
READS 4 OF 16 VALUES
ID
REGION
AMOUNT
TS
2041
2042
2043
2044
APAC
US
EU
US
128.00
16.40
73.25
9.10
03-11
03-11
03-12
03-12
SKIPPED
SKIPPED
SKIPPED
READ PATH — ONE CONTIGUOUS BLOCK: THE ENGINE READS THE AMOUNT COLUMN AND NOTHING ELSE
SAME 4 ROWS, SAME 16 VALUES — ONLY THE ORDER ON DISK CHANGES. A QUERY TOUCHING 1 OF 4 COLUMNS READS 25% OF THE BYTES.