Question

我有一个PostgreSQL字符串数组作为表中的列。我使用GIN方法创建了一个索引。但是任何查询都不会使用索引（相反，它们使用过滤器对整个表进行顺序扫描）。我错过了什么？

这是我的迁移：

class CreateDocuments < ActiveRecord::Migration
  def up
    create_table :documents do |t|
      t.string :title
      t.string :tags, array: true, default: []
      t.timestamps
    end

    add_index :documents, :tags, using: 'gin'

    (1..100000).each do |i|
      tags = []
      tags << 'even' if (i % 2) == 0
      tags << 'odd' if (i % 2) == 1
      tags << 'divisible by 3' if (i % 3) == 0
      tags << 'divisible by 4' if (i % 4) == 0
      tags << 'divisible by 5' if (i % 5) == 0

      Document.create(
        title: i,
        tags: tags
      )
    end
  end

  def down
    drop_table :documents
  end
end

这是我的查询，结果是行数。

Document.where("'divisible by 5' = ANY (tags)").explain
    Document Load (249.8ms)  SELECT "documents".* FROM "documents" WHERE ('divisible by 5' = ANY (tags))
    D, [2014-03-07T17:09:49.689709 #41937] DEBUG -- :   Document Load (249.8ms)  SELECT "documents".* FROM "documents" WHERE ('divisible by 5' = ANY (tags))
    => EXPLAIN for: SELECT "documents".* FROM "documents"  WHERE ('divisible by 5' = ANY (tags))
                       QUERY PLAN
    -----------------------------------------------------------------
    Seq Scan on documents  (cost=0.00..3500.00 rows=20057 width=69)
      Filter: ('divisible by 5'::text = ANY ((tags)::text[]))
    (2 rows)

Document.where("'divisible by 5' = ANY (tags)").length
    Document Load (258.0ms)  SELECT "documents".* FROM "documents" WHERE ('divisible by 5' = ANY (tags))
    D, [2014-03-07T17:09:55.536517 #41937] DEBUG -- :   Document Load (258.0ms)  SELECT "documents".* FROM "documents" WHERE ('divisible by 5' = ANY (tags))
    => 20000

Answer 1

要使用GIN索引，请使用<@ ("is contained by") operator代替ANY construct。

The manual states here默认GIN索引目前仅支持这些运算符（附加功能随扩展一起提供）：

<@
@>
=
&&

请尝试此查询：

Document.where("'{divisible by 5}' <@ tags").explain

请注意，左侧也需要在array notation，即使它是单个元素。运算符<@适用于数组。因此'{divisible by 5}'。

为什么我的PostgreSQL数组索引没有被使用（Rails 4）？

1 个答案: