Indexes

Indexes are a data structure that allow you to speed up your document queries by telling Convex how to organize your documents. Indexes also allow you to change the order of documents in query results.

For a more in-depth introduction to indexing see Indexes and Query Performance.

Defining indexes

Indexes are defined as part of your Convex schema. Each index consists of:

1. A name.
   • Must be unique per table.
2. An ordered list of fields to index.
   • To specify a field on a nested document, use a dot-separated path like properties.name.

To add an index onto a table, use the index method on your table's schema:

convex/schema.ts

import { defineSchema, defineTable } from "convex/server";
import { v } from "convex/values";

// Define a messages table with two indexes.
export default defineSchema({
  messages: defineTable({
    channel: v.id("channels"),
    body: v.string(),
    user: v.id("users"),
  })
    .index("by_channel", ["channel"])
    .index("by_channel_user", ["channel", "user"]),
});

The by_channel index is ordered by the channel field defined in the schema. For messages in the same channel, they are ordered by the system-generated _creationTime field which is added to all indexes automatically.

By contrast, the by_channel_user index orders messages in the same channel by the user who sent them, and only then by _creationTime.

Indexes are created in npx convex dev and npx convex deploy.

You may notice that the first deploy that defines an index is a bit slower than normal. This is because Convex needs to backfill your index. The more data in your table, the longer it will take Convex to organize it in index order. If this is problematic for your workflow, contact us.

You can feel free to query an index in the same deploy that defines it. Convex will ensure that the index is backfilled before the new query and mutation functions are registered.

Be careful when removing indexes

In addition to adding new indexes, npx convex deploy will delete indexes that are no longer present in your schema. Make sure that your indexes are completely unused before removing them from your schema!

Querying documents using indexes

A query for "messages in channel created 1-2 minutes ago" over the by_channel index would look like:

const messages = await ctx.db
  .query("messages")
  .withIndex("by_channel", (q) =>
    q
      .eq("channel", channel)
      .gt("_creationTime", Date.now() - 2 * 60000)
      .lt("_creationTime", Date.now() - 60000)
  )
  .collect();

The .withIndex method defines which index to query and how Convex will use that index to select documents. The first argument is the name of the index and the second is an index range expression. An index range expression is a description of which documents Convex should consider when running the query.

The choice of index both affects how you write the index range expression and what order the results are returned in. For instance, by making both a by_channel and by_channel_user index, we can get results within a channel ordered by _creationTime or by user, respectively. If you were to use the by_channel_user index like this:

const messages = await ctx.db
  .query("messages")
  .withIndex("by_channel_user", (q) => q.eq("channel", channel))
  .collect();

The results would be all of the messages in a channel ordered by user, then by _creationTime. If you were to use by_channel_user like this:

const messages = await ctx.db
  .query("messages")
  .withIndex("by_channel_user", (q) =>
    q.eq("channel", channel).eq("user", user)
  )
  .collect();

The results would be the messages in the given channel sent by user, ordered by _creationTime.

An index range expression is always a chained list of:

1. 0 or more equality expressions defined with .eq.
2. [Optionally] A lower bound expression defined with .gt or .gte.
3. [Optionally] An upper bound expression defined with .lt or .lte.

You must step through fields in index order.

Each equality expression must compare a different index field, starting from the beginning and in order. The upper and lower bounds must follow the equality expressions and compare the next field.

For example, it is not possible to write a query like:

// DOES NOT COMPILE!
const messages = await ctx.db
  .query("messages")
  .withIndex("by_channel", (q) =>
    q
      .gt("_creationTime", Date.now() - 2 * 60000)
      .lt("_creationTime", Date.now() - 60000)
  )
  .collect();

This query is invalid because the by_channel index is ordered by (channel, _creationTime) and this query range has a comparison on _creationTime without first restricting the range to a single channel. Because the index is sorted first by channel and then by _creationTime, it isn't a useful index for finding messages in all channels created 1-2 minutes ago. The TypeScript types within withIndex will guide you through this.

To better understand what queries can be run over which indexes, see Introduction to Indexes and Query Performance.

The performance of your query is based on the specificity of the range.

For example, if the query is

const messages = await ctx.db
  .query("messages")
  .withIndex("by_channel", (q) =>
    q
      .eq("channel", channel)
      .gt("_creationTime", Date.now() - 2 * 60000)
      .lt("_creationTime", Date.now() - 60000)
  )
  .collect();

then query's performance would be based on the number of messages in channel created 1-2 minutes ago.

If the index range is not specified, all documents in the index will be considered in the query.

Picking a good index range

For performance, define index ranges that are as specific as possible! If you are querying a large table and you're unable to add any equality conditions with .eq, you should consider defining a new index.

.withIndex is designed to only allow you to specify ranges that Convex can efficiently use your index to find. For all other filtering you can use the .filter method.

For example to query for "messages in channel not created by me" you could do:

const messages = await ctx.db
  .query("messages")
  .withIndex("by_channel", q => q.eq("channel", channel))
  .filter(q => q.neq(q.field("user"), myUserId)
  .collect();

In this case the performance of this query will be based on how many messages are in the channel. Convex will consider each message in the channel and only return the messages where the user field matches myUserId.

Sorting with indexes

Queries that use withIndex are ordered by the columns specified in the index.

The order of the columns in the index dictates the priority for sorting. The values of the columns listed first in the index are compared first. Subsequent columns are only compared as tie breakers only if all earlier columns match.

Since Convex automatically includes _creationTime as the last column in all indexes, _creationTime will always be the final tie breaker if all other columns in the index are equal.

For example, by_channel_user includes channel, user, and \_creationTime. So queries on messages that use .withIndex("by_channel_user") will be sorted first by channel, then by user within each channel, and finally by the creation time.

Sorting with indexes allows you to satisfy use cases like displaying the top N scoring users, the most recent N transactions, or the the most N liked messages.

For example, to get the top 10 highest scoring players in your game, you might define an index on the player's highest score:

export default defineSchema({
  players: defineTable({
    username: v.string(),
    highestScore: v.number(),
  }).index("by_highest_score", ["highestScore"]),
});

You can then efficiently find the top 10 highest scoring players using your index and take(10):

const topScoringPlayers = await ctx.db
  .query("users")
  .withIndex("by_highest_score")
  .order("desc")
  .take(10);

In this example, the range expression is omitted because we're looking for the highest scoring players of all time. This particular query is reasonably efficient for large data sets only because we're using take().

If you use an index without a range expression, you should always use one of the following in conjunction with withIndex:

1. .first()
2. .unique()
3. .take(n)
4. .paginate(ops)

These APIs allow you to efficiently limit your query to a reasonable size without performing a full table scan.

Full Table Scans

When your query fetches documents from the database, it will scan the rows in the range you specify. If you are using .collect(), for instance, it will scan all of the rows in the range. So if you use withIndex without a range expression, you will be scanning the whole table, which can be slow when your table has thousands of rows. .filter() doesn't affect which documents are scanned. Using .first() or .unique() or .take(n) will only scan rows until it has enough documents.

You can include a range expression to satisfy more targeted queries. For example, to get the top scoring players in Canada, you might use both take() and a range expression:

// query the top 10 highest scoring players in Canada.
const topScoringPlayers = await ctx.db
  .query("users")
  .withIndex("by_country_highest_score", (q) => q.eq("country", "CA"))
  .order("desc")
  .take(10);

Limits

Convex supports indexes containing up to 16 fields. You can define 32 indexes on each table. Indexes can't contain duplicate fields.

No reserved fields (starting with _) are allowed in indexes. The _creationTime field is automatically added to the end of every index to ensure a stable ordering. It should not be added explicitly in the index definition, and it's counted towards the index fields limit.

The by_creation_time index is created automatically (and is what is used in database queries that don't specify an index). The by_id index is reserved.