title: Get the SRID of a geometry field
body: <p>I have a table with some geometry types in and need to be able to return the SRID of a given field. How can I do this. I have had a search but all I can find is the function to alter a SRID. In Oracle I am doing this:</p>
  
  <pre><code>A.CLUSTER_EXTENT.SDO_SRID 
  </code></pre>
  
  <p>Is there an equivalent function in PostGIS? </p>