pg.Cursor

A cursor can be used to efficiently read through large result sets without loading the entire result-set into memory ahead of time. It's useful to simulate a 'streaming' style read of data, or exit early from a large result set. The cursor is passed to client.query and is dispatched internally in a way very similar to how normal queries are sent, but the API it presents for consuming the result set is different.

install

$ npm install pg pg-cursor

constructor

new Cursor(text: String, values: Any[][, config: CursorQueryConfig])

Instantiates a new Cursor. A cursor is an instance of Submittable and should be passed directly to the client.query method.

const { Pool } = require('pg')
const Cursor = require('pg-cursor')
const pool = new Pool()
const client = await pool.connect()
const text = 'SELECT * FROM my_large_table WHERE something > $1'
const values = [10]
const cursor = client.query(new Cursor(text, values))
cursor.read(100, (err, rows) => {
  cursor.close(() => {
    client.release()
  })
})

interface CursorQueryConfig {
  // by default rows come out as a key/value pair for each row
  // pass the string 'array' here to receive rows as an array of values
  rowMode?: string;
  // custom type parsers just for this query result
  types?: Types;
}

read

cursor.read(rowCount: Number, callback: (err: Error, rows: Row[], result: pg.Result) => void) => void

Read rowCount rows from the cursor instance. The callback will be called when the rows are available, loaded into memory, parsed, and converted to JavaScript types.

If the cursor has read to the end of the result sets all subsequent calls to cursor#read will return a 0 length array of rows. Calling read on a cursor that has read to the end.

Here is an example of reading to the end of a cursor:

const { Pool } = require('pg')
const Cursor = require('pg-cursor')
const pool = new Pool()
const client = await pool.connect()
const cursor = client.query(new Cursor('select * from generate_series(0, 5)'))
cursor.read(100, (err, rows) => {
  if (err) {
    throw err
  }
  assert(rows.length == 6)
  cursor.read(100, (err, rows) => {
    assert(rows.length == 0)
  })
})

close

cursor.close(callback: () => void) => void

Used to close the cursor early. If you want to stop reading from the cursor before you get all of the rows returned, call this.