FW: indirect dereferencing a field in a record using plpgsql - Mailing list pgsql-hackers
From | Conrad Vermeulen |
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Subject | FW: indirect dereferencing a field in a record using plpgsql |
Date | |
Msg-id | FKEMIMFAIHJKGEBKMNNNCEILCEAA.conrad@fastforward.za.net Whole thread Raw |
Responses |
Re: FW: indirect dereferencing a field in a record using plpgsql
(Tom Lane <tgl@sss.pgh.pa.us>)
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List | pgsql-hackers |
Hi, I had a problem where I needed to indirectly dereference a field from a record. To illustrate: CREATE FUNCTION test2() returns bool as ' DECLAREmyrec record;fld text; BEGINselect ''hello'' as a, ''world'' as b into myrec; fld = ''a''; -- the fieldname from myrec that we are interested in raise exception ''the result is "% %"'', myrec._fld, myrec.b;return true; END; ' language 'plpgsql2'; Typically, dereferencing is done by specifying the fieldname that was obtained either by a previous query as illustrated above, or in predefined structures such as in the cases of triggers with the NEW and OLD records. The problem I had was that the functionality described above was not available. Building up a query string and then using the 'EXECUTE' functionality also does not work as I think a new context is created and the 'NEW' and 'OLD' records are not available - or they are part of the plpgsql language and not available via the methods when calling EXECUTE. Requirements for such functionality is probably quite limited in general. Most applications you would know what columns you are dealing with, but the above requirement could add some flexability. I noticed this was possible to do under pltcl, but plpgsql did not support this. Would this be something worth adding to plpgsql? Anyways, I decided to hack the plpgsql language a little - for my testing I created a seperate language 'plpgsql2' which had my hack. The patch I made was on the pl_exec.c file on the postgresql 7.3.2 source - the diff of the original to my hack is listed below. I realized I still have a problem. Apparently there are issues with types with regards to the planner. I only discovered this when I tried accessing the NEW and OLD records in a trigger. The problem that arose was that the type that is being dereferenced is not necessarily what is expected when compared with the type that the planner planned. The problem I faced was in the function exec_stmt_execsql(): if (expr->plan_argtypes[i] != SPI_gettypeid(rec->tupdesc, fno)) elog(ERROR, "type of %s.%s doesn't match that whenpreparing the plan", rec->refname, recfield->fieldname); I have to spend some more time understanding the exact ADT that is created by the parser. Possibly my quick hack may not work if the parser predetermines the type as the functionality required would only really be able to determine the type at time of execution. If anyone can give some insight to help make the functionality work - would be appreciated. I'll have to spend some more time figuring out the internals I guess. :) Thanks, Conrad Vermeulen conrad@fastforward.za.net To briefly comment the extenstion I added: Normal field dereferencing is done by 'record.field' where 'field' is part of the 'record'. In cases where 'indirect dereferencing' is required, the 'field' is actually a variable which needs to be dereferenced first. In order to differenciate between a normal record field, and a global field, my extension requires a '_' before the field name to identify that this process is required. The use of the '_' was because the parser tokenized other tokens such as $ or % and I did not want to patch any other code. # diff pl_exec.orig.c pl_exec.c 1737a1738,1772 > > > #define DYNAMDTYPE_RECFIELD \ > if (fno == SPI_ERROR_NOATTRIBUTE) \ >\ > int j; \ > char *fld; \ > if (recfield->fieldname[0] == '_') \ >\ > fld= recfield->fieldname+1; \ > for (j=0;j<estate->ndatums; j++) \ >\ > var = (PLpgSQL_var *) (estate->datums[j]); \ > if ((estate->datums[j]->dtype == PLPGSQL_DTYPE_VAR) && \ > (!strcmp(fld,var->refname))) \ >\ > if (var->isnull ) \ >\ > elog(ERROR, "\"%s\" found but not assigned for use by \"%s\"", fld, rec->refname); \ > break; \ > } \ > fld=DatumGetCString(DirectFunctionCall1(textout,var->value)); \ > fno = SPI_fnumber(rec->tupdesc, fld); \ > pfree(fld); \ > if (fno != SPI_ERROR_NOATTRIBUTE) \ > break; \ > } \ > } \ > } \ > if (fno == SPI_ERROR_NOATTRIBUTE) \ > elog(ERROR, "record \"%s\" has no field named \"%s\"", rec->refname, recfield->fieldname); \ > } > > > 1821,1822c1856 < if (fno == SPI_ERROR_NOATTRIBUTE) < elog(ERROR, "record \"%s\" has no field named \"%s\"", rec->refname, recfield->fieldname); --- > DYNAMDTYPE_RECFIELD 1997,1998c2031 < if (fno == SPI_ERROR_NOATTRIBUTE) < elog(ERROR, "record \"%s\" has no field named \"%s\"", rec->refname, recfield->fieldname); --- > DYNAMDTYPE_RECFIELD 2083,2085c2116 < if (fno == SPI_ERROR_NOATTRIBUTE) < elog(ERROR, "record \"%s\" has no field named \"%s\"", rec->refname, recfield->fieldname); < --- > DYNAMDTYPE_RECFIELD 2644,2645c2675 < if (fno == SPI_ERROR_NOATTRIBUTE) < elog(ERROR, "record \"%s\" has no field named \"%s\"", rec->refname, recfield->fieldname); --- > DYNAMDTYPE_RECFIELD 2938,2939c2968 < if (fno == SPI_ERROR_NOATTRIBUTE) < elog(ERROR, "record \"%s\" has no field named \"%s\"", rec->refname, recfield->fieldname); --- > DYNAMDTYPE_RECFIELD 3132,3133c3161 < if (fno == SPI_ERROR_NOATTRIBUTE) < elog(ERROR, "record \"%s\" has no field named \"%s\"", rec->refname, recfield->fieldname); --- > DYNAMDTYPE_RECFIELD 3270,3271c3298 < if (fno == SPI_ERROR_NOATTRIBUTE) < elog(ERROR, "record \"%s\" has no field named \"%s\"", rec->refname, recfield->fieldname); --- > DYNAMDTYPE_RECFIELD
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