Why is it so much faster passing an array as a literal rather than a parameter?

Question

I have a query like the following:

SELECT
   table1.field1,
   table1.field2,
   table3.field1,
   table3.field2,
   table3.field3,
   table3.field4

FROM table1

INNER JOIN table2
ON table1.field1 = table2.field1

INNER JOIN table3
ON table2.field2 = table3.field5

WHERE table1.field1 = ANY(@pArray);

All of these comparisons are with string/text fields. There are indexes on all of the foreign key fields. This is a poorly designed database that I have mostly just read-only access to.

What I'm wondering is why the above parameter causes this query to take 10+ seconds, whereas if I pass the array manually as an array literal (using string concatenation), then it takes about 40-50 milliseconds.

My array contains 195 8-character numeric string elements.

My only guess is that it has something to do with sanitizing the parameter values, but making the query take hundreds of times longer seems a bit excessive. What I've done is add my own check to make sure that all of the strings are 100% numeric, which I think should avoid SQL injection attacks.

My NpgSql code looks like this:

void MyQuery(
   string[] pArray
) {
   var sql = @"
      // as above
   ";

   using var connection = new NpgsqlConnection(mConnectionString);
   connection.Open();

   using var command = new NpgsqlCommand(sql, connection);
   command.Parameters.AddWithValue("pArray", pArray);

   // takes ~12 seconds with parameter, or ~50ms with array literal
   using var reader = command.ExecuteReader();

When removing the parameter, the WHERE clause ends up looking more like this, after concating:

WHERE table1.field1 = ANY('{12345678,12345679,12345680,12345681}');

Is the slowness due only to sanitization, or a bug in NpgSql, or something else?

Answer 1

The problem as indicated in the comments was adding a parameter without specifying the type. Changing the parameter line to the following solved the problem:

command.Parameters.AddWithValue("pArray", NpgsqlDbType.Array | NpgsqlDbType.Char, pArray);

Notice that to specify an array type, you OR the Array value with another value, in this case Char . The actual PostgreSQL type in the database is something like character varying(15) .