如何理解index skip scan

問別人什麼是index skip scan的時候，很大回答都侷限於，這個執行計劃不常見，而且出如今有組合索引，可是前導列再也不謂語條件中的查詢。僅此而已。

這樣的回答，確實基本回答了index skip scan的使用場景。

Index skip scans improve index scans by nonprefix columns. Often, scanning index blocks is faster than scanning table data blocks.

The database may choose an index skip scan when the leading column of the composite index is not specified in a query predicate.

但沒有解釋，index skip scan是如何實現，在沒有前導列的狀況下，進行"skip" scan的。

Skip scanning lets a composite index be split logically into smaller subindexes. In skip scanning, the initial column of the composite index is not specified in the query. In other words, it is skipped.

In a skip scan, the number of logical subindexes is determined by the number of distinct values in the leading column.

看到這裏，其實就比較好理解了。邏輯上，經過前導列，把組合索引分紅幾個更小子索引，而後對這些子因此進行掃描，併合並結果。子索引的數目，由前導列的不一樣值的數目決定。下面是一個例子：

-- For example, assume that you run the following query for a customer in the sh.customers table:

SELECT * FROM sh.customers WHERE cust_email = 'Abbey@company.com';

-- The customers table has a column cust_gender whose values are either M or F. Assume that a composite index exists on the columns (cust_gender, cust_email) that was created as follows:

CREATE INDEX customers_gender_email ON sh.customers (cust_gender, cust_email);

-- portion of the index entries.
F,Wolf@company.com,rowid
F,Wolsey@company.com,rowid
F,Wood@company.com,rowid
F,Woodman@company.com,rowid
F,Yang@company.com,rowid
F,Zimmerman@company.com,rowid
M,Abbassi@company.com,rowid
M,Abbey@company.com,rowid

-- The database can use a skip scan of this index even though cust_gender is not specified in the WHERE clause.

-- When searching for the record for the customer whose email is  bbey@company.com, the database searches the subindex with the value First and then searches the subindex with the value M. Conceptually, the database processes the query as follows:

SELECT * FROM sh.customers WHERE cust_gender = 'F' 
  AND cust_email = 'Abbey@company.com'
UNION ALL
SELECT * FROM sh.customers WHERE cust_gender = 'M'
  AND cust_email = 'Abbey@company.com';