复合索引下getAll的条件设置，及用get替代filter的等效查询

Hey there! Let's break down your questions one by one:

1. 复合索引场景下getAll方法的条件设置 #

The core principle here is aligning your query conditions with the order of the composite index—this is how you get the most out of index optimization.

Let's say you've defined a composite index (to, from) on your emails table. Here's how to structure your getAll conditions properly:

• To fetch all records matching both to and from, specify the fields in the exact order of the index:

  db.emails.getAll({ to: "targetRecipient", from: "targetSender" });
  

• You can also use a prefix of the index to retrieve a broader dataset (this still leverages the index efficiently):

  // Gets all emails sent to "targetRecipient", no matter who the sender is
  db.emails.getAll({ to: "targetRecipient" });
  

• Avoid skipping the prefix fields (like querying only from without to), as this will bypass the composite index and force a full table scan, which is much slower.

Also, stick to exact matches whenever possible. If you need to use range queries (like > or <), apply them only to the last field in the composite index to keep the index usable.

2. Equivalent get Method Query for Your SQL Statement #

Your SQL query select * from emails where to="someemail" and from="some@email" targets records with exact matches on two fields. To replicate this with a get method (instead of filter), you first need a composite unique index on (to, from)—since get is designed to fetch a single record (or the first match) using a unique identifier or unique index.

Once that index is set up, here's the equivalent get call:

db.emails.get({ to: "someemail", from: "some@email" });

This works because:

• It matches both fields in the exact order of the composite index, just like your SQL WHERE clause.
• The unique index ensures there’s at most one record matching both conditions (if you expect multiple records, note that get will only return the first one—so adjust your index or expectations accordingly).
• It’s way more efficient than filter because it uses the index to directly locate the record, instead of scanning every entry in the table.

内容的提问来源于stack exchange，提问作者nicky