fix: Improvements to SET search_path for postgres explain plans (#5422)

### Brief description of Pull Request
During dogfooding we discovered some cases where explain plans were
failing to run due to missing or ambiguous schema names.

This PR addresses several small issues with the `SET search_path`
command which is meant to address these issues.

### Pull Request Details

- Sets the search path first, before any other requests are issued,
including creating the prepared statement.
- Places the datname in double quotes to prevent syntax errors
- Explicitly sets the search path for SESSION rather than depending upon
the default.

### PR Checklist
- [x] Tests updated

Author: rgeyer

internal/component/database_observability/postgres/collector/explain_plans.go

@@ -568,6 +568,11 @@ func (c *ExplainPlans) fetchExplainPlanJSON(ctx context.Context, qi queryInfo) (
 	}
 	defer conn.Close()
 
+	setSearchPathStatement := fmt.Sprintf("SET SESSION search_path TO \"%s\", public", qi.datname)
+	if _, err := conn.ExecContext(ctx, setSearchPathStatement); err != nil {
+		return nil, fmt.Errorf("failed to set search path: %w", err)
+	}
+
 	preparedStatementName := strings.ReplaceAll(fmt.Sprintf("explain_plan_%s", qi.queryId), "-", "_")
 	preparedStatementText := fmt.Sprintf("PREPARE %s AS %s", preparedStatementName, qi.queryText)
 	logger := log.With(c.logger, "query_id", qi.queryId, "datname", qi.datname, "preparedStatementName", preparedStatementName, "preparedStatementText", preparedStatementText)
@@ -581,11 +586,6 @@ func (c *ExplainPlans) fetchExplainPlanJSON(ctx context.Context, qi queryInfo) (
 		}
 	}()
 
-	setSearchPathStatement := fmt.Sprintf("SET search_path TO %s, public", qi.datname)
-	if _, err := conn.ExecContext(ctx, setSearchPathStatement); err != nil {
-		return nil, fmt.Errorf("failed to set search path: %w", err)
-	}
-
 	if _, err := conn.ExecContext(ctx, "SET plan_cache_mode = force_generic_plan"); err != nil {
 		return nil, fmt.Errorf("failed to set plan cache mode: %w", err)
 	}

internal/component/database_observability/postgres/collector/explain_plans_test.go

@@ -2915,8 +2915,8 @@ func TestExplainPlanFetchExplainPlans(t *testing.T) {
 			require.Equal(t, "complex_aggregation_with_case.json", jsonFile.Name)
 			jsonData := jsonFile.Data
 
+			mock.ExpectExec("SET SESSION search_path TO \"testdb\", public").WillReturnResult(sqlmock.NewResult(0, 1))
 			mock.ExpectExec("PREPARE explain_plan_123456 AS select * from some_table where id = $1").WillReturnResult(sqlmock.NewResult(0, 1))
-			mock.ExpectExec("SET search_path TO testdb, public").WillReturnResult(sqlmock.NewResult(0, 1))
 			mock.ExpectExec("SET plan_cache_mode = force_generic_plan").WillReturnResult(sqlmock.NewResult(0, 1))
 			mock.ExpectQuery("EXPLAIN (FORMAT JSON) EXECUTE explain_plan_123456(null)").WillReturnRows(sqlmock.NewRows([]string{"json"}).AddRow(jsonData))
 			mock.ExpectExec("DEALLOCATE explain_plan_123456").WillReturnResult(sqlmock.NewResult(0, 1))
@@ -2982,8 +2982,8 @@ func TestExplainPlanFetchExplainPlans(t *testing.T) {
 			require.Equal(t, "complex_aggregation_with_case.json", jsonFile.Name)
 			jsonData := jsonFile.Data
 
+			mock.ExpectExec("SET SESSION search_path TO \"testdb\", public").WillReturnResult(sqlmock.NewResult(0, 1))
 			mock.ExpectExec("PREPARE explain_plan_123456 AS with cte as (select * from some_table where id = $1) select * from cte").WillReturnResult(sqlmock.NewResult(0, 1))
-			mock.ExpectExec("SET search_path TO testdb, public").WillReturnResult(sqlmock.NewResult(0, 1))
 			mock.ExpectExec("SET plan_cache_mode = force_generic_plan").WillReturnResult(sqlmock.NewResult(0, 1))
 			mock.ExpectQuery("EXPLAIN (FORMAT JSON) EXECUTE explain_plan_123456(null)").WillReturnRows(sqlmock.NewRows([]string{"json"}).AddRow(jsonData))
 			mock.ExpectExec("DEALLOCATE explain_plan_123456").WillReturnResult(sqlmock.NewResult(0, 1))
@@ -3049,8 +3049,8 @@ func TestExplainPlanFetchExplainPlans(t *testing.T) {
 			require.Equal(t, "complex_aggregation_with_case.json", jsonFile.Name)
 			jsonData := jsonFile.Data
 
+			mock.ExpectExec("SET SESSION search_path TO \"testdb\", public").WillReturnResult(sqlmock.NewResult(0, 1))
 			mock.ExpectExec("PREPARE explain_plan_123456 AS select * from some_table").WillReturnResult(sqlmock.NewResult(0, 1))
-			mock.ExpectExec("SET search_path TO testdb, public").WillReturnResult(sqlmock.NewResult(0, 1))
 			mock.ExpectExec("SET plan_cache_mode = force_generic_plan").WillReturnResult(sqlmock.NewResult(0, 1))
 			mock.ExpectQuery("EXPLAIN (FORMAT JSON) EXECUTE explain_plan_123456").WillReturnRows(sqlmock.NewRows([]string{"json"}).AddRow(jsonData))
 			mock.ExpectExec("DEALLOCATE explain_plan_123456").WillReturnResult(sqlmock.NewResult(0, 1))