> If I remember, described functionality was implemented in early patches, > but was removed to simplify code. To now, there was not a request to do it. > > Unfortunately, the documentation was not fixed.
I'll do that, as I'm working in there anyway. :)
> column expressions are evaluated once per row, but XPath expression is > compiled per row too, if I remember well.
I looked for evidence of that in the code, but did not find it; the compilation appears to happen in XmlTableSetColumnFilter, which is called from tfuncInitialize.
it is called per input row.
I can't guarantee I didn't miss something, though.
> We designed it more tolerant as > we expected possibility to store XPath expression in one column and data in > second column.
Perhaps if I could see an example showing the functionality... The nearest I could find in the regression tests was the test with COLUMNS a int PATH '' || lower(_path) || 'c' but, again, I think that test only demonstrates how LATERAL works, not any behavior specific to xmltable.
the main reason for this was not to support support some specific patterns - just used design doesn't requires stronger changes in executor or internal multi call caching of some internal data. Probably you can find discussion related to this topic in mailing list archive.
sure - using any expression in PATH clause should to demonstrate this functionality.