NPGSQL: Streaming results from Postgres query?
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19-09-2019 - |
Question
I have a large table (2,000,000 rows) and I'd like to print each record to the screen, one at time, without loading the entire table into memory.
//pseudo code
var cmd = new NpgSQLCommand();
cmd.CommandText = "SELECT * FROM mytable;"
IReader reader = cmd.ExecuteReader(); //blocks until the entire set is returned
while(reader.Read()) //ideally each call to read loads more results from the db.
{
// print record name
}
So as noted in the code above, the ExecuteReader() doesn't continue until the entire set is loaded into memory. How do I change this behavior so the results are streamed?
Thanks
ETA: While this seems like homework, it's not. It's just an easier way to describe a problem that involves reading an entire table with a single query but processing the results a row at a time.
ETA x2:
From npgsql
Warning: There is a known issue when calling ExecuteReader and large tables. Currently Version 1 of Npgsql gets all data from table before returning. If you are experiencing bad performance in such cases, you may need to use a server cursor to page through rows. For that, you can use a code like the following:
Solution 2
Okay, well it looks like this is a known issue with npgsql 1.0:
The workaround is to use a server cursor:
using System;
using System.Data;
using Npgsql;
public static class NpgsqlUserManual
{
public static void Main(String[] args)
{
NpgsqlConnection conn = new NpgsqlConnection("Server=127.0.0.1;Port=5432;User Id=joe;Password=secret;Database=joedata;");
conn.Open();
NpgsqlCommand command = new NpgsqlCommand("select version()", conn);
String serverversion;
try
{
serverversion = (String)command.ExecuteScalar();
Console.WriteLine("PostgreSQL server version: {0}", serverversion);
}
finally
{
conn.Close();
}
}
}
OTHER TIPS
Npgsql2 now handles large resultsets much better. It doesn't load all data to memory. So, you don't need to use a server side cursor anymore.
I hope it helps.
Francisco Figueiredo Jr. Npgsql Lead Developer
The easiest way to do this is:
COPY ( select * from Table) TO STDOUT
Check the syntax of copy to see how to print it in .csv or any other format if you need to...
To print out the whole table with column name and value use following code:
using (var conn = new NpgsqlConnection("Host=IPADDRESS;Username=USER;Password=PASS;Database=DBNAME;"))
{
conn.Open();
using (var cmd = new NpgsqlCommand())
{
cmd.Connection = conn;
cmd.CommandText = "SELECT * FROM TABLE_NAME";
StringBuilder str;
int count;
using (var reader = cmd.ExecuteReader())
{
while (reader.Read())
{
str = new StringBuilder();
count = 0;
while (count < reader.FieldCount) {
str.Append(reader.GetName(count) + ": " + reader.GetValue(count));
if ((count + 1) < reader.FieldCount)
{
str.Append(", ");
}
count++;
}
Console.WriteLine(str.ToString());
Console.WriteLine("====================");
}
}
}