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Review: Re: [PATCH] Re: [HACKERS] Adding xpath_exists function

From: David Fetter <david(at)fetter(dot)org>
To: Mike Fowler <mike(at)mlfowler(dot)com>
Cc: Bruce Momjian <bruce(at)momjian(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>,pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>,Round Robin Reviewers <pgsql-rrreviewers(at)postgresql(dot)org>
Subject: Review: Re: [PATCH] Re: [HACKERS] Adding xpath_exists function
Date: 2010-07-27 23:33:25
Message-ID: 20100727233325.GO26815@fetter.org (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-rrreviewers
== Submission review ==

* Is the patch in context diff format?

    Yes.

* Does it apply cleanly to the current CVS HEAD?

    Yes.

    patch -p1 < ../xpath_exists-3.patch 
    patching file doc/src/sgml/func.sgml
    Hunk #1 succeeded at 8642 (offset 16 lines).
    patching file src/backend/utils/adt/xml.c
    patching file src/include/catalog/pg_proc.h
    Hunk #1 succeeded at 4391 (offset 6 lines).
    patching file src/include/utils/xml.h
    patching file src/test/regress/expected/xml.out
    patching file src/test/regress/sql/xml.sql

* Does it include reasonable tests, necessary doc patches, etc?

    Tests:

        As this is new functionality, it doesn't really need to test
        much for interactions with other parts of the system.

        I'm not really an XML expert, so I'd like to punt as to
        whether it tests enough functionality.

        Minor quibble with the regression tests: should we be using
        dollar quotes in things like this?  Doubled-up quote marks:

        SELECT xpath_exists('//town[text() = ''Cwmbran'']','<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>'::xml);

        Dollar quote:

        SELECT xpath_exists($$//town[text() = 'Cwmbran']$$,'<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>'::xml);


    Doc patches: Good up to cross-Atlantic differences in spelling
    (speciali[sz]ed), e.g.

== Usability review ==  

Read what the patch is supposed to do, and consider:

* Does the patch actually implement that? 

    Yes.

* Do we want that? 

    Yes.

* Do we already have it? 

    Not really.  There are kludges to accomplish these things, but
    they're available mostly in the sense that a general-purpose
    language allows you to write code to do anything a Turing machine
    can do.

* Does it follow SQL spec, or the community-agreed behavior? 

    Yes.

* Does it include pg_dump support (if applicable)?

    Not applicable.

* Are there dangers? 

    Not that I can see.

* Have all the bases been covered?

    To the extent of my XML knowledge, yes.

== Feature test ==

Apply the patch, compile it and test:

* Does the feature work as advertised?

    Yes.

* Are there corner cases the author has failed to consider?

    Not that I've found.  See above re: XML and my vast ignorance of
    same.

* Are there any assertion failures or crashes?

    No.

== Performance review ==

* Does the patch slow down simple tests? 

    No.

* If it claims to improve performance, does it?

    No such claim made.  The kludges needed to reproduce the
    functionality would certainly consume an enormous number of
    developer hours, though.

* Does it slow down other things?

    Not that I've found.  There might be some minuscule slowing down
    of the code to the existence of more code paths, but we're a long,
    long way from having that be something other than noise.

== Coding review ==

Read the changes to the code in detail and consider:

* Does it follow the project [http://developer.postgresql.org/pgdocs/postgres/source.html coding guidelines]? 

    Yes.

* Are there portability issues? 

    Not that I can see.

* Will it work on Windows/BSD etc? 

    Should do.

* Are the comments sufficient and accurate?

    Yes.

* Does it do what it says, correctly?

    Yes, subject to, etc.

* Does it produce compiler warnings?

    No.

* Can you make it crash?

    No.

== Architecture review ==

Consider the changes to the code in the context of the project as a whole:

* Is everything done in a way that fits together coherently with other features/modules? 

    Yes.

* Are there interdependencies that can cause problems?

    Not that I've found.

Cheers,
David.
On Tue, Jun 29, 2010 at 11:37:28AM +0100, Mike Fowler wrote:
> Mike Fowler wrote:
> >Bruce Momjian wrote:
> >>I have added this to the next commit-fest:
> >>
> >>    https://commitfest.postgresql.org/action/commitfest_view?id=6
> >Thanks Bruce. Attached is a revised patch which changes the code
> >slightly such that it uses an older version of the libxml library.
> >I've added comments to the code so that we remember why we didn't
> >use the latest function.
> 
> After seeing some other posts in the last couple of days, I realised
> I hadn't documented the function in the SGML. I have now done so,
> and added a couple of tests with XML literals. Please find the patch
> attached. Now time to go correct the xmlexists patch too...
> 
> -- 
> Mike Fowler
> Registered Linux user: 379787
> 

> *** a/doc/src/sgml/func.sgml
> --- b/doc/src/sgml/func.sgml
> ***************
> *** 8626,8631 **** SELECT xpath('/my:a/text()', '<my:a xmlns:my="http://example.com">test</my:a>',
> --- 8626,8664 ----
>   (1 row)
>   ]]></screen>
>      </para>
> +    
> +    <sect3>
> +     <title>xpath_exists</title>
> +     
> +     <indexterm>
> +      <primary>xpath_exists</primary>
> +     </indexterm>
> +     
> +     <synopsis>
> +      <function>xpath_exists</function>(<replaceable>xpath</replaceable>, <replaceable>xml</replaceable><optional>, <replaceable>nsarray</replaceable></optional>)
> +     </synopsis>
> +     
> +     <para>
> +      The function <function>xpath_exists</function> is a specialised form
> +      of the <function>xpath</function> function. Though the functions are
> +      syntactically the same the xpath expressions are evaluated in differing
> +      contexts. Instead of returning the XML values that satisfy the xpath, this
> +      function returns a boolean indicating whether the query was satisfied or not.
> +     </para>
> +     
> +     <para>
> +      Example:
> +      <screen><![CDATA[
> + SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com">test</my:a>', 
> +                     ARRAY[ARRAY['my', 'http://example.com']]);
> + 
> +  xpath_exists  
> + ------------
> +  t
> + (1 row)
> + ]]></screen>
> +     </para>
> +    </sect3>
>     </sect2>
>   
>     <sect2 id="functions-xml-mapping">
> *** a/src/backend/utils/adt/xml.c
> --- b/src/backend/utils/adt/xml.c
> ***************
> *** 3495,3497 **** xpath(PG_FUNCTION_ARGS)
> --- 3495,3681 ----
>   	return 0;
>   #endif
>   }
> + 
> + /*
> +  * Determines if the node specified by the supplied XPath exists
> +  * in a given XML document, returning a boolean.
> +  *
> +  * It is up to the user to ensure that the XML passed is in fact
> +  * an XML document - XPath doesn't work easily on fragments without
> +  * a context node being known.
> +  */
> + Datum
> + xpath_exists(PG_FUNCTION_ARGS)
> + {
> + #ifdef USE_LIBXML
> + 	text	   *xpath_expr_text = PG_GETARG_TEXT_P(0);
> + 	xmltype    *data = PG_GETARG_XML_P(1);
> + 	ArrayType  *namespaces = PG_GETARG_ARRAYTYPE_P(2);
> + 	ArrayBuildState *astate = NULL;
> + 	xmlParserCtxtPtr ctxt = NULL;
> + 	xmlDocPtr	doc = NULL;
> + 	xmlXPathContextPtr xpathctx = NULL;
> + 	xmlXPathCompExprPtr xpathcomp = NULL;
> + 	xmlXPathObjectPtr xpathobj = NULL;
> + 	char	   *datastr;
> + 	int32		len;
> + 	int32		xpath_len;
> + 	xmlChar    *string;
> + 	xmlChar    *xpath_expr;
> + 	int			i;
> + 	int			res_nitems;
> + 	int			ndim;
> + 	Datum	   *ns_names_uris;
> + 	bool	   *ns_names_uris_nulls;
> + 	int			ns_count;
> + 	int			result;
> + 
> + 	/*
> + 	 * Namespace mappings are passed as text[].  If an empty array is passed
> + 	 * (ndim = 0, "0-dimensional"), then there are no namespace mappings.
> + 	 * Else, a 2-dimensional array with length of the second axis being equal
> + 	 * to 2 should be passed, i.e., every subarray contains 2 elements, the
> + 	 * first element defining the name, the second one the URI.  Example:
> + 	 * ARRAY[ARRAY['myns', 'http://example.com'], ARRAY['myns2',
> + 	 * 'http://example2.com']].
> + 	 */
> + 	ndim = ARR_NDIM(namespaces);
> + 	if (ndim != 0)
> + 	{
> + 		int		   *dims;
> + 
> + 		dims = ARR_DIMS(namespaces);
> + 
> + 		if (ndim != 2 || dims[1] != 2)
> + 			ereport(ERROR,
> + 					(errcode(ERRCODE_DATA_EXCEPTION),
> + 					 errmsg("invalid array for XML namespace mapping"),
> + 					 errdetail("The array must be two-dimensional with length of the second axis equal to 2.")));
> + 
> + 		Assert(ARR_ELEMTYPE(namespaces) == TEXTOID);
> + 
> + 		deconstruct_array(namespaces, TEXTOID, -1, false, 'i',
> + 						  &ns_names_uris, &ns_names_uris_nulls,
> + 						  &ns_count);
> + 
> + 		Assert((ns_count % 2) == 0);	/* checked above */
> + 		ns_count /= 2;			/* count pairs only */
> + 	}
> + 	else
> + 	{
> + 		ns_names_uris = NULL;
> + 		ns_names_uris_nulls = NULL;
> + 		ns_count = 0;
> + 	}
> + 
> + 	datastr = VARDATA(data);
> + 	len = VARSIZE(data) - VARHDRSZ;
> + 	xpath_len = VARSIZE(xpath_expr_text) - VARHDRSZ;
> + 	if (xpath_len == 0)
> + 		ereport(ERROR,
> + 				(errcode(ERRCODE_DATA_EXCEPTION),
> + 				 errmsg("empty XPath expression")));
> + 
> + 	string = (xmlChar *) palloc((len + 1) * sizeof(xmlChar));
> + 	memcpy(string, datastr, len);
> + 	string[len] = '\0';
> + 
> + 	xpath_expr = (xmlChar *) palloc((xpath_len + 1) * sizeof(xmlChar));
> + 	memcpy(xpath_expr, VARDATA(xpath_expr_text), xpath_len);
> + 	xpath_expr[xpath_len] = '\0';
> + 
> + 	pg_xml_init();
> + 	xmlInitParser();
> + 
> + 	PG_TRY();
> + 	{
> + 		/*
> + 		 * redundant XML parsing (two parsings for the same value during one
> + 		 * command execution are possible)
> + 		 */
> + 		ctxt = xmlNewParserCtxt();
> + 		if (ctxt == NULL)
> + 			xml_ereport(ERROR, ERRCODE_OUT_OF_MEMORY,
> + 						"could not allocate parser context");
> + 		doc = xmlCtxtReadMemory(ctxt, (char *) string, len, NULL, NULL, 0);
> + 		if (doc == NULL)
> + 			xml_ereport(ERROR, ERRCODE_INVALID_XML_DOCUMENT,
> + 						"could not parse XML document");
> + 		xpathctx = xmlXPathNewContext(doc);
> + 		if (xpathctx == NULL)
> + 			xml_ereport(ERROR, ERRCODE_OUT_OF_MEMORY,
> + 						"could not allocate XPath context");
> + 		xpathctx->node = xmlDocGetRootElement(doc);
> + 		if (xpathctx->node == NULL)
> + 			xml_ereport(ERROR, ERRCODE_INTERNAL_ERROR,
> + 						"could not find root XML element");
> + 
> + 		/* register namespaces, if any */
> + 		if (ns_count > 0)
> + 		{
> + 			for (i = 0; i < ns_count; i++)
> + 			{
> + 				char	   *ns_name;
> + 				char	   *ns_uri;
> + 
> + 				if (ns_names_uris_nulls[i * 2] ||
> + 					ns_names_uris_nulls[i * 2 + 1])
> + 					ereport(ERROR,
> + 							(errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
> + 					  errmsg("neither namespace name nor URI may be null")));
> + 				ns_name = TextDatumGetCString(ns_names_uris[i * 2]);
> + 				ns_uri = TextDatumGetCString(ns_names_uris[i * 2 + 1]);
> + 				if (xmlXPathRegisterNs(xpathctx,
> + 									   (xmlChar *) ns_name,
> + 									   (xmlChar *) ns_uri) != 0)
> + 					ereport(ERROR,		/* is this an internal error??? */
> + 							(errmsg("could not register XML namespace with name \"%s\" and URI \"%s\"",
> + 									ns_name, ns_uri)));
> + 			}
> + 		}
> + 
> + 		xpathcomp = xmlXPathCompile(xpath_expr);
> + 		if (xpathcomp == NULL)	/* TODO: show proper XPath error details */
> + 			xml_ereport(ERROR, ERRCODE_INTERNAL_ERROR,
> + 						"invalid XPath expression");
> + 
> + 		/* Version 2.6.27 introduces a function named xmlXPathCompiledEvalToBoolean
> + 		 * however we can derive the existence by whether any nodes are returned
> + 		 * thereby preventing a library version upgrade */
> + 		xpathobj = xmlXPathCompiledEval(xpathcomp, xpathctx);
> + 		if (xpathobj == NULL)	/* TODO: reason? */
> + 			xml_ereport(ERROR, ERRCODE_INTERNAL_ERROR,
> + 						"could not create XPath object");
> + 
> + 		if (xpathobj->nodesetval == NULL)
> + 			result = 0;
> + 		else
> + 			result = xpathobj->nodesetval->nodeNr;
> + 	}
> + 	PG_CATCH();
> + 	{
> + 		if (xpathobj)
> + 			xmlXPathFreeObject(xpathobj);
> + 		if (xpathcomp)
> + 			xmlXPathFreeCompExpr(xpathcomp);
> + 		if (xpathctx)
> + 			xmlXPathFreeContext(xpathctx);
> + 		if (doc)
> + 			xmlFreeDoc(doc);
> + 		if (ctxt)
> + 			xmlFreeParserCtxt(ctxt);
> + 		PG_RE_THROW();
> + 	}
> + 	PG_END_TRY();
> + 
> + 	xmlXPathFreeCompExpr(xpathcomp);
> + 	xmlXPathFreeContext(xpathctx);
> + 	xmlFreeDoc(doc);
> + 	xmlFreeParserCtxt(ctxt);
> + 
> + 	PG_RETURN_BOOL(result);
> + #else
> + 	NO_XML_SUPPORT();
> + 	return 0;
> + #endif
> + }
> *** a/src/include/catalog/pg_proc.h
> --- b/src/include/catalog/pg_proc.h
> ***************
> *** 4385,4390 **** DESCR("evaluate XPath expression, with namespaces support");
> --- 4385,4395 ----
>   DATA(insert OID = 2932 (  xpath		 PGNSP PGUID 14 1 0 0 f f f t f i 2 0 143 "25 142" _null_ _null_ _null_ _null_ "select pg_catalog.xpath($1, $2, ''{}''::pg_catalog.text[])" _null_ _null_ _null_ ));
>   DESCR("evaluate XPath expression");
>   
> + DATA(insert OID = 3037 (  xpath_exists	 PGNSP PGUID 12 1 0 0 f f f t f i 3 0 16 "25 142 1009" _null_ _null_ _null_ _null_ xpath_exists _null_ _null_ _null_ ));
> + DESCR("evaluate XPath expression in a boolean context, with namespaces support");
> + DATA(insert OID = 3038 (  xpath_exists	 PGNSP PGUID 14 1 0 0 f f f t f i 2 0 16 "25 142" _null_ _null_ _null_ _null_ "select pg_catalog.xpath_exists($1, $2, ''{}''::pg_catalog.text[])" _null_ _null_ _null_ ));
> + DESCR("evaluate XPath expression in a boolean context");
> + 
>   /* uuid */
>   DATA(insert OID = 2952 (  uuid_in		   PGNSP PGUID 12 1 0 0 f f f t f i 1 0 2950 "2275" _null_ _null_ _null_ _null_ uuid_in _null_ _null_ _null_ ));
>   DESCR("I/O");
> *** a/src/include/utils/xml.h
> --- b/src/include/utils/xml.h
> ***************
> *** 37,42 **** extern Datum texttoxml(PG_FUNCTION_ARGS);
> --- 37,43 ----
>   extern Datum xmltotext(PG_FUNCTION_ARGS);
>   extern Datum xmlvalidate(PG_FUNCTION_ARGS);
>   extern Datum xpath(PG_FUNCTION_ARGS);
> + extern Datum xpath_exists(PG_FUNCTION_ARGS);
>   
>   extern Datum table_to_xml(PG_FUNCTION_ARGS);
>   extern Datum query_to_xml(PG_FUNCTION_ARGS);
> *** a/src/test/regress/expected/xml.out
> --- b/src/test/regress/expected/xml.out
> ***************
> *** 502,504 **** SELECT xpath('//b', '<a>one <b>two</b> three <b>etc</b></a>');
> --- 502,557 ----
>    {<b>two</b>,<b>etc</b>}
>   (1 row)
>   
> + -- Test xpath_exists evaluation
> + SELECT xpath_exists('//town[text() = ''Toronto'']','<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>'::xml);
> +  xpath_exists 
> + --------------
> +  f
> + (1 row)
> + 
> + SELECT xpath_exists('//town[text() = ''Cwmbran'']','<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>'::xml);
> +  xpath_exists 
> + --------------
> +  t
> + (1 row)
> + 
> + INSERT INTO xmltest VALUES (4, '<menu><beers><name>Budvar</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);
> + INSERT INTO xmltest VALUES (5, '<menu><beers><name>Molson</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);
> + INSERT INTO xmltest VALUES (6, '<myns:menu xmlns:myns="http://myns.com"><myns:beers><myns:name>Budvar</myns:name><myns:cost>free</myns:cost><myns:name>Carling</myns:name><myns:cost>lots</myns:cost></myns:beers></myns:menu>'::xml);
> + INSERT INTO xmltest VALUES (7, '<myns:menu xmlns:myns="http://myns.com"><myns:beers><myns:name>Molson</myns:name><myns:cost>free</myns:cost><myns:name>Carling</myns:name><myns:cost>lots</myns:cost></myns:beers></myns:menu>'::xml);
> + SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/menu/beer',data);
> +  count 
> + -------
> +      0
> + (1 row)
> + 
> + SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/menu/beers',data);
> +  count 
> + -------
> +      2
> + (1 row)
> + 
> + SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/menu/beers/name[text() = ''Molson'']',data);
> +  count 
> + -------
> +      1
> + (1 row)
> + 
> + SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/myns:menu/myns:beer',data,ARRAY[ARRAY['myns','http://myns.com']]);
> +  count 
> + -------
> +      0
> + (1 row)
> + 
> + SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/myns:menu/myns:beers',data,ARRAY[ARRAY['myns','http://myns.com']]);
> +  count 
> + -------
> +      2
> + (1 row)
> + 
> + SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/myns:menu/myns:beers/myns:name[text() = ''Molson'']',data,ARRAY[ARRAY['myns','http://myns.com']]);
> +  count 
> + -------
> +      1
> + (1 row)
> + 
> *** a/src/test/regress/sql/xml.sql
> --- b/src/test/regress/sql/xml.sql
> ***************
> *** 163,165 **** SELECT xpath('', '<!-- error -->');
> --- 163,181 ----
>   SELECT xpath('//text()', '<local:data xmlns:local="http://127.0.0.1"><local:piece id="1">number one</local:piece><local:piece id="2" /></local:data>');
>   SELECT xpath('//loc:piece/@id', '<local:data xmlns:local="http://127.0.0.1"><local:piece id="1">number one</local:piece><local:piece id="2" /></local:data>', ARRAY[ARRAY['loc', 'http://127.0.0.1']]);
>   SELECT xpath('//b', '<a>one <b>two</b> three <b>etc</b></a>');
> + 
> + -- Test xpath_exists evaluation
> + SELECT xpath_exists('//town[text() = ''Toronto'']','<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>'::xml);
> + SELECT xpath_exists('//town[text() = ''Cwmbran'']','<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>'::xml);
> + 
> + INSERT INTO xmltest VALUES (4, '<menu><beers><name>Budvar</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);
> + INSERT INTO xmltest VALUES (5, '<menu><beers><name>Molson</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);
> + INSERT INTO xmltest VALUES (6, '<myns:menu xmlns:myns="http://myns.com"><myns:beers><myns:name>Budvar</myns:name><myns:cost>free</myns:cost><myns:name>Carling</myns:name><myns:cost>lots</myns:cost></myns:beers></myns:menu>'::xml);
> + INSERT INTO xmltest VALUES (7, '<myns:menu xmlns:myns="http://myns.com"><myns:beers><myns:name>Molson</myns:name><myns:cost>free</myns:cost><myns:name>Carling</myns:name><myns:cost>lots</myns:cost></myns:beers></myns:menu>'::xml);
> + 
> + SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/menu/beer',data);
> + SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/menu/beers',data);
> + SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/menu/beers/name[text() = ''Molson'']',data);
> + SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/myns:menu/myns:beer',data,ARRAY[ARRAY['myns','http://myns.com']]);
> + SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/myns:menu/myns:beers',data,ARRAY[ARRAY['myns','http://myns.com']]);
> + SELECT COUNT(id) FROM xmltest WHERE xpath_exists('/myns:menu/myns:beers/myns:name[text() = ''Molson'']',data,ARRAY[ARRAY['myns','http://myns.com']]);

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