如何在聚合查询中使用索引？

Question

给定这样的查询

SELECT franchise, MAX(worth)
FROM figurines
GROUP BY franchise

什么样的索引可以加快查询速度，数据库将如何使用该索引？

如果需要更多详细信息，请假设列 franchise 的基数相对较低，而 worth 的基数非常高。

我个人正在使用 mysql，但我正在寻找对算法的一般理解，而不是特定于供应商的实现细节。

Answer 1

场景一：无索引（阅读全文table）

foreach(page in table.pages)
{
  foreach(row in page.rows)
  {
    Compare and accumulate franchise and worth from row
  }
}
-- Total IO = table.pages

情形 2：仅特许经营指数

foreach(page in index.pages)
{
  foreach(indexRow in page.rows)
  {
    tableRow = table.fetchRow(indexRow); // + 1 page of IO for each row
    Compare and accumulate franchise from indexRow and worth from tableRow
  }
}
-- Total IO = index.pages + table.rows
-- this is likely to be greater than Scenario 1...
--  so optimizer should prefer that plan instead.

场景 3：按顺序覆盖指数（特许经营权、价值）。

foreach(page in index.pages)
{
  foreach(row in page.rows)
  {
    Compare and accumulate franchise and worth from row
  }
}
-- Total IO = index.pages
-- Assuming that index is thinner than table, a win!

场景 4：使用场景 3 中索引的已知特许经营列表的不同查询

foreach(franchise in franchises)
{
  SELECT MAX(worth) FROM figurines WHERE franchise = franchise
}

...

foreach(franchise in franchises)
{
  search into the index looking for the last record with this franchise
  // this is usually less than 10 pages of IO in my experience.
}
-- Total IO = count of franchise * 10
-- super win!

场景 4 不同，因为它记录的是查找而不是扫描。

如何在聚合查询中使用索引？

How is an index used in an query with aggregation?

sql

indexing

group-by

aggregate-functions