Add pqerror package

Add a pqerror package with a list of error codes. Typical usage might be
something like:

	pqErr, ok := errors.AsType[*pq.Error](err)
	if ok && pqErr == pqerror.UniqueViolation {
		return fmt.Errorf("user already exists")
	}

To make this a bit more convenient, it also adds a pqerror.As()
function:

	pqErr := pqerror.As(err, pqerror.UniqueViolation)
	if pqErr != nil {
		log.Fatalf("email %q already exsts", email)
	}
	if err != nil {
		return err
	}

This also moves most of the error stuff to the pqerror package, with
type aliases in the pq package so it won't break anything. Has to export
Error.Query to make this work, which is okay. Keep most of the tests in
the main pq package for now, to ensure this works.

Fixes 492

Author: arp242

CHANGELOG.md

@@ -8,6 +8,22 @@ unreleased
 
 - Support `sslmode=prefer` and `sslmode=allow` ([#1270]).
 
+- Add a new `pqerror` package with PostgreSQL error codes ([#1275]).
+
+  For example, to test if an error is a UNIQUE constraint violation:
+
+      pqErr, ok := pq.AsType[*pq.Error](err)
+      if ok && pqErr.Code == pqerror.UniqueViolation {
+          log.Fatalf("email %q already exsts", email)
+      }
+
+  To make this a bit more convenient, it also adds a pqerror.As() function:
+
+      pqErr := pqerror.As(err, pqerror.UniqueViolation)
+      if pqErr != nil {
+          log.Fatalf("email %q already exsts", email)
+      }
+
 ### Fixes
 
 - Fix SSL key permission check to allow modes stricter than 0600/0640#1265 ([#1265]).
@@ -23,6 +39,7 @@ unreleased
 [#1267]: https://github.com/lib/pq/pull/1267
 [#1270]: https://github.com/lib/pq/pull/1270
 [#1272]: https://github.com/lib/pq/pull/1272
+[#1275]: https://github.com/lib/pq/pull/1275
 
 v1.11.2 (2026-02-10)
 --------------------
@@ -134,6 +151,8 @@ newer. Previously PostgreSQL 8.4 and newer were supported.
 
 - Handle ErrorResponse in readReadyForQuery and return proper error ([#1136]).
 
+- Detect COPY even if the query starts with whitespace or comments ([#1198]).
+
 - CopyIn() and CopyInSchema() now work if the list of columns is empty, in which
   case it will copy all columns ([#1239]).
 
@@ -159,6 +178,7 @@ newer. Previously PostgreSQL 8.4 and newer were supported.
 [#1180]: https://github.com/lib/pq/pull/1180
 [#1184]: https://github.com/lib/pq/pull/1184
 [#1188]: https://github.com/lib/pq/pull/1188
+[#1198]: https://github.com/lib/pq/pull/1198
 [#1211]: https://github.com/lib/pq/pull/1211
 [#1212]: https://github.com/lib/pq/pull/1212
 [#1214]: https://github.com/lib/pq/pull/1214

deprecated.go

@@ -11,49 +11,6 @@ type PGError interface {
 	Get(k byte) (v string)
 }
 
-// Get implements the legacy PGError interface.
-//
-// Deprecated: new code should use the fields of the Error struct directly.
-func (e *Error) Get(k byte) (v string) {
-	switch k {
-	case 'S':
-		return e.Severity
-	case 'C':
-		return string(e.Code)
-	case 'M':
-		return e.Message
-	case 'D':
-		return e.Detail
-	case 'H':
-		return e.Hint
-	case 'P':
-		return e.Position
-	case 'p':
-		return e.InternalPosition
-	case 'q':
-		return e.InternalQuery
-	case 'W':
-		return e.Where
-	case 's':
-		return e.Schema
-	case 't':
-		return e.Table
-	case 'c':
-		return e.Column
-	case 'd':
-		return e.DataTypeName
-	case 'n':
-		return e.Constraint
-	case 'F':
-		return e.File
-	case 'L':
-		return e.Line
-	case 'R':
-		return e.Routine
-	}
-	return ""
-}
-
 // ParseURL converts a url to a connection string for driver.Open.
 //
 // Deprecated: directly passing an URL to sql.Open("postgres", "postgres://...")