Fix for count being very slow on large Parse Classes' collections (Postgres) (#5330)

* Changed count to be approximate. Should help with postgres slowness

* refactored last commit to only fall back to estimate if no complex query

* handlign variables correctly

* Trying again because it was casting to lowercase table names which doesnt work for us/

* syntax error

* Adding quotations to pg query

* hopefully final pg fix

* Postgres will now use an approximate count unless there is a more complex query specified

* handling edge case

* Fix for count being very slow on large Parse Classes' collections in Postgres. Replicating fix for Mongo in issue 5264

* Fixed silly spelling error resulting from copying over notes

* Lint fixes

* limiting results to 1 on approximation

* suppress test that we can no longer run for postgres

* removed tests from Postgres that no longer apply

* made changes requested by dplewis

* fixed count errors

* updated package.json

* removed test exclude for pg

* removed object types from method

* test disabled for postgres

* returned type

* add estimate count test

* fix mongo test

src/Adapters/Storage/Postgres/PostgresStorageAdapter.js

@@ -1962,17 +1962,37 @@ export class PostgresStorageAdapter implements StorageAdapter {
   }
 
   // Executes a count.
-  count(className: string, schema: SchemaType, query: QueryType) {
-    debug('count', className, query);
+  count(
+    className: string,
+    schema: SchemaType,
+    query: QueryType,
+    readPreference?: string,
+    estimate?: boolean = true
+  ) {
+    debug('count', className, query, readPreference, estimate);
     const values = [className];
     const where = buildWhereClause({ schema, query, index: 2 });
     values.push(...where.values);
 
     const wherePattern =
       where.pattern.length > 0 ? `WHERE ${where.pattern}` : '';
-    const qs = `SELECT count(*) FROM $1:name ${wherePattern}`;
+    let qs = '';
+
+    if (where.pattern.length > 0 || !estimate) {
+      qs = `SELECT count(*) FROM $1:name ${wherePattern}`;
+    } else {
+      qs =
+        'SELECT reltuples AS approximate_row_count FROM pg_class WHERE relname = $1';
+    }
+
     return this._client
-      .one(qs, values, a => +a.count)
+      .one(qs, values, a => {
+        if (a.approximate_row_count != null) {
+          return +a.approximate_row_count;
+        } else {
+          return +a.count;
+        }
+      })
       .catch(error => {
         if (error.code !== PostgresRelationDoesNotExistError) {
           throw error;
@@ -2327,6 +2347,11 @@ export class PostgresStorageAdapter implements StorageAdapter {
   updateSchemaWithIndexes(): Promise<void> {
     return Promise.resolve();
   }
+
+  // Used for testing purposes
+  updateEstimatedCount(className: string) {
+    return this._client.none('ANALYZE $1:name', [className]);
+  }
 }
 
 function convertPolygonToSQL(polygon) {