@graphile/pg-aggregates

Adds a powerful suite of aggregate functionality to a PostGraphile schema: calculating aggregates, grouped aggregates, applying conditions to grouped aggregates, ordering by relational aggregates, filtering by the results of aggregates on related connections, etc.

IMPORTANT: aggregates are added to connections, they do not work with "simple collections".

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Status

This module is currently "experimental" status; we may change any part of it in a semver minor release.

Usage

Requires PostGraphile v4.12.0-alpha.0 or higher.

Install with:

yarn add postgraphile @graphile/pg-aggregates

CLI usage via --append-plugins:

postgraphile --append-plugins @graphile/pg-aggregates -c postgres://localhost/my_db ...

Library usage via appendPlugins:

import PgAggregatesPlugin from "@graphile/pg-aggregates";
// or: const PgAggregatesPlugin = require("@graphile/pg-aggregates").default;

const middleware = postgraphile(DATABASE_URL, SCHEMAS, {
  appendPlugins: [PgAggregatesPlugin],
});

If you want you could install our example schema and then issue a GraphQL query such as:

query GameAggregates {
  allMatchStats {
    aggregates {
      max {
        points
        goals
        saves
      }
      min {
        points
      }
    }
  }
  allPlayers(orderBy: [MATCH_STATS_BY_PLAYER_ID_SUM_GOALS_ASC]) {
    nodes {
      name
      matchStatsByPlayerId {
        totalCount
        aggregates {
          sum {
            points
            goals
            saves
          }
          average {
            points
            goals
            saves
            teamPosition
          }
        }
      }
    }
  }
}

or:

query GroupedAggregatesByDerivative {
  allMatchStats {
    byDay: groupedAggregates(groupBy: [CREATED_AT_TRUNCATED_TO_DAY]) {
      keys # The timestamp truncated to the beginning of the day
      average {
        points
      }
    }
    byHour: groupedAggregates(groupBy: [CREATED_AT_TRUNCATED_TO_HOUR]) {
      keys # The timestamp truncated to the beginning of the hour
      average {
        points
      }
    }
  }
}

To filter by aggregates on related tables, you will also need postgraphile-plugin-connection-filter, and you will need to enable graphileBuildOptions.connectionFilterRelations as documented here.

app.use(
  postgraphile(DATABASE_URL, SCHEMA_NAME, {
    graphileBuildOptions: {
      connectionFilterRelations: true,
    },
  })
);

Interaction with connection parameters

Aggregates respect the conditions/filters of the connection but are unaffected by the pagination of the connection (they ignore the first/last/after/before/orderBy parameters). You may retrieve (optionally paginated) node data from a connection at the same time as retrieving aggregates from it. Aggregates are supported on connections at any level of the GraphQL query.

Aggregates

Connection-wide aggregates are available via the aggregates field directly on a GraphQL connection; for example:

query LoadsOfAggregates {
  allFilms {
    aggregates {
      average {
        durationInMinutes
      }
    }
  }
}

We support the following aggregates out of the box:

• sum (applies to number-like fields) - the result of adding all the values together
• distinctCount (applies to all fields) - the count of the number of distinct values
• min (applies to number-like fields) - the smallest value
• max (applies to number-like fields) - the greatest value
• average (applies to number-like fields) - the average (arithmetic mean) value
• stddevSample (applies to number-like fields) - the sample standard deviation of the values
• stddevPopulation (applies to number-like fields) - the population standard deviation of the values
• varianceSample (applies to number-like fields) - the sample variance of the values
• variancePopulation (applies to number-like fields) - the population variance of the values

See Defining your own aggregates below for details on how to add your own aggregates.

Different aggregates apply to different data types; in general we attempt to add aggregate entries for each column and computed column function that appears to be compatible with the aggregate.

Ordering by aggregates

This plugin automatically adds some additional orderBy criteria to your graph allowing you to order by aggregates over relations; e.g. you could find the top 5 players ordered by their average points scored in each match, and grab some more aggregate information about them too:

query FocussedOrderedAggregate {
  allPlayers(
    first: 5
    orderBy: [MATCH_STATS_BY_PLAYER_ID_AVERAGE_POINTS_DESC]
  ) {
    nodes {
      name
      matchStatsByPlayerId {
        totalCount
        aggregates {
          sum {
            goals
          }
          average {
            points
          }
        }
      }
    }
  }
}

Filtering by aggregates

(You will need postgraphile-plugin-connection-filter for this; see above.)

query PlayersWith9OrMoreSavesInMatchesTheyScoredIn {
  allPlayers(
    filter: {
      matchStatsByPlayerId: {
        aggregates: {
          sum: { saves: { greaterThan: "9" }, rating: { lessThan: 143 } }
          filter: { goals: { greaterThan: 0 } }
        }
      }
    }
  ) {
    nodes {
      name
      matchStatsByPlayerId(filter: { goals: { greaterThan: 0 } }) {
        aggregates {
          sum {
            saves
            rating
            goals
          }
        }
      }
    }
  }
}

Grouped aggregates

We also support grouping your data via the value of one of your columns or a derivative thereof; and calculating aggregates over each of the matching groups. Out of the box we support two derivatives:

• truncated-to-hour (applies to timestamp-like values) - truncates to the beginning of the (UTC) hour
• truncated-to-day (applies to timestamp-like values) - truncates to the beginning of the (UTC) day

See Defining your own grouping derivatives below for details on how to add your own grouping derivatives.

The aggregates supported over groups are the same as over the connection as a whole (see Aggregates above), but in addition you may also determine the keys that were used for the aggregate. There will be one key for each of the groupBy values; for example in this query:

query AverageDurationByYearOfRelease {
  allFilms {
    groupedAggregates(groupBy: [YEAR_OF_RELEASE]) {
      keys
      average {
        durationInMinutes
      }
    }
  }
}

each entry in the groupedAggregates result will have a keys entry that will be a list containing one value which will be the year of release (as a string). The values in the keys list are always stringified, this is a known limitation due to interactions with GraphQL.

Having

If these grouped aggregates are returning too much data, you can filter the groups down by applying a having clause against them; for example you could see the average number of goals on days where the average points score was over 200:

query AverageGoalsOnDaysWithAveragePointsOver200 {
  allMatchStats {
    byDay: groupedAggregates(
      groupBy: [CREATED_AT_TRUNCATED_TO_DAY]
      having: { average: { points: { greaterThan: 200 } } }
    ) {
      keys
      average {
        goals
      }
    }
  }
}

Defining your own aggregates

You can add your own aggregates by using a plugin to add your own aggregate specs. Aggregate specs aren't too complicated, for example here is a spec that could define the "min" aggregate:

const isNumberLike = (pgType) => pgType.category === "N";

const minSpec = {
  id: "min",
  humanLabel: "minimum",
  HumanLabel: "Minimum",
  isSuitableType: isNumberLike,
  sqlAggregateWrap: (sqlFrag) => sql.fragment`min(${sqlFrag})`,
};

See src/AggregateSpecsPlugin.ts for more details/examples.

Defining your own grouping derivatives

You may add your own derivatives by adding a group by spec to build.pgAggregateGroupBySpecs via a plugin. Derivative specs are fairly straightforward, for example here's the spec for "truncated-to-hour":

const TIMESTAMP_OID = "1114";
const TIMESTAMPTZ_OID = "1184";

const truncatedToHourSpec = {
  // A unique identifier for this spec, will be used to generate its name:
  id: "truncated-to-hour",

  // A filter to determine which column/function return types this derivative
  // is valid against:
  isSuitableType: (pgType) =>
    pgType.id === TIMESTAMP_OID || pgType.id === TIMESTAMPTZ_OID,

  // The actual derivative - given the SQL fragment `sqlFrag` which represents
  // the column/function call, return a new SQL fragment that represents the
  // derived value, in this case a truncated timestamp:
  sqlWrap: (sqlFrag) => sql.fragment`date_trunc('hour', ${sqlFrag})`,
};

Building that up with a few more different intervals into a full PostGraphile plugin, you might write something like:

// Constants from PostgreSQL
const TIMESTAMP_OID = "1114";
const TIMESTAMPTZ_OID = "1184";

// Determine if a given type is a timestamp/timestamptz
const isTimestamp = (pgType) =>
  pgType.id === TIMESTAMP_OID || pgType.id === TIMESTAMPTZ_OID;

// Build a spec that truncates to the given interval
const tsTruncateSpec = (sql, interval) => ({
  // `id` has to be unique, derive it from the `interval`:
  id: `truncated-to-${interval}`,

  // Only apply to timestamp fields:
  isSuitableType: isTimestamp,

  // Given the column value represented by the SQL fragment `sqlFrag`, wrap it
  // with a `date_trunc()` call, passing the relevant interval.
  sqlWrap: (sqlFrag) =>
    sql.fragment`date_trunc(${sql.literal(interval)}, ${sqlFrag})`,
});

// This is the PostGraphile plugin; see:
// https://www.graphile.org/postgraphile/extending/
const DateTruncAggregateGroupSpecsPlugin = (builder) => {
  builder.hook("build", (build) => {
    const { pgSql: sql } = build;

    build.pgAggregateGroupBySpecs = [
      // Copy all existing specs, except the ones we're replacing
      ...build.pgAggregateGroupBySpecs.filter(
        (spec) => !["truncated-to-day", "truncated-to-hour"].includes(spec.id)
      ),

      // Add our timestamp specs
      tsTruncateSpec(sql, "year"),
      tsTruncateSpec(sql, "month"),
      tsTruncateSpec(sql, "week"),
      tsTruncateSpec(sql, "day"),
      tsTruncateSpec(sql, "hour"),
      // Other values: microseconds, milliseconds, second, minute, quarter,
      // decade, century, millennium.
      // See https://www.postgresql.org/docs/current/functions-datetime.html#FUNCTIONS-DATETIME-TRUNC
    ];

    return build;
  });
};

module.exports = DateTruncAggregateGroupSpecsPlugin;

Finally pass this plugin into PostGraphile via --append-plugins or appendPlugins: [...] - see https://www.graphile.org/postgraphile/extending/

See src/AggregateSpecsPlugin.ts for examples and more information.

Disable aggregates

By default, aggregates are created for all tables. This significantly increases the size of your GraphQL schema, and could also be a security (DoS) concern as aggregates can be expensive. We recommend that you use the disableAggregatesByDefault: true option to disable aggregates by default, and then enable them only for the tables you need:

const middleware = postgraphile(DATABASE_URL, SCHEMAS, {
  // ...
  appendPlugins: [
    // ...
    PgAggregatesPlugin,
  ],

  graphileBuildOptions: {
    // Disable aggregates by default; opt each table in via the `@aggregates` smart tag
    disableAggregatesByDefault: true,
  },
});

Enable aggregates for a specific table:

"class": {
  "my_schema.my_table": {
    "tags": {
      "aggregates": "on"
    }
  }
}

or:

COMMENT ON TABLE my_schema.my_table IS E'@aggregates on';

You also can keep aggregates enabled by default, but disable aggregates for specific tables:

"class": {
  "my_schema.my_table": {
    "tags": {
      "aggregates": "off"
    }
  }
}

or:

COMMENT ON TABLE my_schema.my_table IS E'@aggregates off';

Thanks

This plugin was started as a proof of concept in 2019 thanks to sponsorship from OneGraph, and was made into fully featured released module thanks to sponsorship from Surge in 2021. It is maintained thanks to the support of Graphile's sponsors - thank you sponsors!