# Other development considerations There are a few considerations to keep in mind as you design your data service and transformation. * You cannot `JOIN` one data service virtual table to another. * Pentaho Data Services uses the Memory Group by step to group. This step keeps all the groups in memory to avoid sorts that can slow down the data service. But, if you plan to use many groups, watch your memory consumption on the server to make sure you don't exceed its limits. * You cannot specify the same field twice in the same `SELECT` clause. * Calculations and functions like string concatenation are not supported. But, you can do these things in the data service transformation. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.pentaho.com/pdia-data-integration/9.3-data-integration/advanced-topics-pentaho-data-integration-overview/pentaho-data-services/test-a-pentaho-data-service/examine-test-results/pentaho-data-service-sql-support-reference-and-other-development-considerations/other-development-considerations.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.