This can be seen when you try to scan the left column of labels.
这一点在你需要扫视左列标签时可以表现出来。
2
As shown in the IXSCAN descriptor in Figure 8, the leading column of the index exploited by optimizer is PRODUCT_NUMBER, which does not provide the order required for merge scan join.
The scan accumulates a word until a break (either the end of a row or column, or a non-letter tile) and then adds the word to all the letter cells it found along the way.