[YSQL] Cleanup PgStatement* casts

[d-uspenskiy]

Jira Link: DB-13845

Description

There are lot of boiler plate code in pggate.cc related to PgStatement* cast.

Status PgApiImpl::AlterTableAddColumn(PgStatement *handle, const char *name,
                                      int order,
                                      const YBCPgTypeEntity *attr_type,
                                      YBCPgExpr missing_value) {
  if (!PgStatement::IsValidStmt(handle, StmtOp::STMT_ALTER_TABLE)) {
    // Invalid handle.
    return STATUS(InvalidArgument, "Invalid statement handle");
  }

  PgAlterTable *pg_stmt = down_cast<PgAlterTable*>(handle);
  return pg_stmt->AddColumn(name, attr_type, order, missing_value);
}

Status PgApiImpl::AlterTableRenameColumn(PgStatement *handle, const char *oldname,
                                         const char *newname) {
  if (!PgStatement::IsValidStmt(handle, StmtOp::STMT_ALTER_TABLE)) {
    // Invalid handle.
    return STATUS(InvalidArgument, "Invalid statement handle");
  }

  PgAlterTable *pg_stmt = down_cast<PgAlterTable*>(handle);
  return pg_stmt->RenameColumn(oldname, newname);
}

It seem reasonable to introduce template helper function GetStatementAs<...> to make the code above look like

Status PgApiImpl::AlterTableAddColumn(
    PgStatement *handle, const char *name, int order, const YBCPgTypeEntity *attr_type,
    YBCPgExpr missing_value) {
  return VERIFY_RESULT_REF(GetStatementAs<PgAlterTable>(handle)).AddColumn(
      name, attr_type, order, missing_value);
}

Status PgApiImpl::AlterTableRenameColumn(
    PgStatement *handle, const char *oldname, const char *newname) {
  return VERIFY_RESULT_REF(GetStatementAs<PgAlterTable>(handle)).RenameColumn(oldname, newname);
}

Issue Type

kind/enhancement

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