Tuesday, August 4, 2009

postgres - xml out (part 3 of 3) table_to_xml

Earlier in the 'postgres xml out' articles we used query_to_xml() to get data out of postgres in xml format.

Postgres also has the function table_to_xml() which is perfect for exporting the whole table in xml format.

Here is the function signature from the postgres documentation page:
table_to_xml(tbl regclass, nulls boolean, tableforest boolean, targetns text)

The first parameter will be 'amd_bang_per_watt' for all the examples in this article.
The fourth parameter I will not be making use of and so '' will be supplied.

The parameters that I will discuss (and vary for illustration) are the boolean second and third parameters, nulls and tableforest respectively.

Four examples of this output are listed below marked with the values I used for the booleans null and tableforest:
  1. amd_bang_per_watt-table_to_xmlFFheaderless.xml (tbl,False,False,'')
  2. amd_bang_per_watt-table_to_xmlFTheaderless.xml (tbl,False,True,'')
  3. amd_bang_per_watt-table_to_xmlTFheaderless.xml (tbl,True,False,'') ***
  4. amd_bang_per_watt-table_to_xmlTTheaderless.xml (tbl,True,True,'')
Option 3 has been marked with *** to indicate this is my preferred output form and I will be using the xml this form produces in the next paragraphs and in some later articles.

Using True for the parameter
nulls boolean
...gets you...
<voltage xsi:nil="true"/>
rather than omission (the voltage node being omitted altogether).

Using False for the parameter
tableforest boolean
means that each row will be wrapped in a
<row> and </row>
pairing which is nice and convenient.

Executing suitable queries in psql and chopping off the head and tail will produce files similar to the four files listed above. An example of *** (Option 3) is shown here:

psql amd_power_dissipation -c "select table_to_xml('amd_bang_per_watt',true,false,'');" | \
tail -n +3 | head -n -3 > amd_bang_per_watt-table_to_xmlTF.xml;

...and the commands in full are in this text file.

Extracting the 1st row, 47th row, and 50th (final) row of sample data from the file amd_bang_per_watt-table_to_xmlTFheaderless.xml, I create row1headerless.xml containing:
<model_family>X2 II </model_family>
<model>550 Black Edition </model>
<l2cache>2x512k </l2cache>
<l3cache>6MB </l3cache>
<voltage>1.15-1.425 </voltage>
<socket>AM3 </socket>
<process_comments>45nm Callisto Q3-2009 </process_comments>
and row47headerless.xml containing:
<model_family>X4 II </model_family>
<model>965 Black Edition </model>
<l2cache>4x512K </l2cache>
<l3cache>6MB </l3cache>
<voltage>1.15-1.425 </voltage>
<socket>AM3 </socket>
<process_comments>x¹ 45nm Deneb Q3-2009 </process_comments>
and row50headerless.xml containing:
<model_family>FX </model_family>
<model>FX-82 x¹ </model>
<l2cache>4x512KB </l2cache>
<l3cache>2MB </l3cache>
<voltage xsi:nil="true"/>
<socket>AM2+ </socket>

You might recall from previous postings regarding this sample data, that the data contains a superscript numeral character 'superscript one' (¹) having utf8 code 00B9 (sometimes appearing in code analysis output as \xc2\xb9).

In particular just glancing at the rows we just output I can see that row47 and row50 both contain superscript one in some of their fields.

The next post will attempt to deal with getting xml data into postgres from a file.

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