Texas Code of Criminal Procedure - CRIM P Art. 28.03. Process for testimony on pleadings

When the matters involved in any written pleading depend in whole or in part upon testimony, and not altogether upon the record of the court, every process known to the law may be obtained on behalf of either party to procure such testimony;  but there shall be no delay on account of the want of the testimony, unless it be shown to the satisfaction of the court that all the means given by the law have been used to procure the same.