Question regarding result.rowCount

[MelissaSnell]

Hi there

I just wanted to clarify something in the documentation.

On this page for the Result object you say that rowCount is not set for the number of rows returned from a query.

I am finding that for a pool.query, rowCount does seem to be > 0 if result.rows.length>0.

Does rowCount reflect the number of rows returned for a pool.query but not for a client.query?

Many thanks in advance.

[sehrope]

The rowCount is populated from the command tag supplied by the PostgreSQL server. It's generally of the form: COMMAND [OID] [ROWS]

Here's the piece of pg that parses the value into the rowCount field:

node-postgres/packages/pg/lib/result.js

Lines 42 to 52 in 3532880

	if (match) {
	this.command = match[1]
	if (match[3]) {
	// COMMMAND OID ROWS
	this.oid = parseInt(match[2], 10)
	this.rowCount = parseInt(match[3], 10)
	} else if (match[2]) {
	// COMMAND ROWS
	this.rowCount = parseInt(match[2], 10)
	}
	}

For DML commands (INSERT, UPDATE, etc), it reflects how many rows the server modified to process the command. For SELECT or COPY commands it reflects how many rows were retrieved or copied. More info on the specifics here: https://www.postgresql.org/docs/current/protocol-message-formats.html (search for CommandComplete for the message type)

The note in the docs about the difference is because that value is controlled by the server. It's possible for a non-standard server (ex: PostgreSQL fork) or a server version in the future to provide different information in some situations so it'd be best not to rely on it to assume that the rows array length matches the rowCount. It's fine to use it for DML counts though.

[MelissaSnell]

Many thanks for the explanation. Much appreciated

[brianc]

holy smokes this description is great - mind if I add it to the docs w/ credit @sehrope ?

[sehrope]

Thanks! Sure go for it!