/*------------------------------------------------------------------------- * * execCurrent.c * executor support for WHERE CURRENT OF cursor * * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * * src/backend/executor/execCurrent.c * *------------------------------------------------------------------------- */ #include "postgres.h" #include "access/genam.h" #include "access/relscan.h" #include "access/sysattr.h" #include "catalog/pg_type.h" #include "executor/executor.h" #include "utils/builtins.h" #include "utils/lsyscache.h" #include "utils/portal.h" #include "utils/rel.h" static char *fetch_cursor_param_value(ExprContext *econtext, int paramId); static ScanState *search_plan_tree(PlanState *node, Oid table_oid, bool *pending_rescan); /* * execCurrentOf * * Given a CURRENT OF expression and the OID of a table, determine which row * of the table is currently being scanned by the cursor named by CURRENT OF, * and return the row's TID into *current_tid. * * Returns true if a row was identified. Returns false if the cursor is valid * for the table but is not currently scanning a row of the table (this is a * legal situation in inheritance cases). Raises error if cursor is not a * valid updatable scan of the specified table. */ bool execCurrentOf(CurrentOfExpr *cexpr, ExprContext *econtext, Oid table_oid, ItemPointer current_tid) { char *cursor_name; char *table_name; Portal portal; QueryDesc *queryDesc; /* Get the cursor name --- may have to look up a parameter reference */ if (cexpr->cursor_name) cursor_name = cexpr->cursor_name; else cursor_name = fetch_cursor_param_value(econtext, cexpr->cursor_param); /* Fetch table name for possible use in error messages */ table_name = get_rel_name(table_oid); if (table_name == NULL) elog(ERROR, "cache lookup failed for relation %u", table_oid); /* Find the cursor's portal */ portal = GetPortalByName(cursor_name); if (!PortalIsValid(portal)) ereport(ERROR, (errcode(ERRCODE_UNDEFINED_CURSOR), errmsg("cursor \"%s\" does not exist", cursor_name))); /* * We have to watch out for non-SELECT queries as well as held cursors, * both of which may have null queryDesc. */ if (portal->strategy != PORTAL_ONE_SELECT) ereport(ERROR, (errcode(ERRCODE_INVALID_CURSOR_STATE), errmsg("cursor \"%s\" is not a SELECT query", cursor_name))); queryDesc = portal->queryDesc; if (queryDesc == NULL || queryDesc->estate == NULL) ereport(ERROR, (errcode(ERRCODE_INVALID_CURSOR_STATE), errmsg("cursor \"%s\" is held from a previous transaction", cursor_name))); /* * We have two different strategies depending on whether the cursor uses * FOR UPDATE/SHARE or not. The reason for supporting both is that the * FOR UPDATE code is able to identify a target table in many cases where * the other code can't, while the non-FOR-UPDATE case allows use of WHERE * CURRENT OF with an insensitive cursor. */ if (queryDesc->estate->es_rowmarks) { ExecRowMark *erm; Index i; /* * Here, the query must have exactly one FOR UPDATE/SHARE reference to * the target table, and we dig the ctid info out of that. */ erm = NULL; for (i = 0; i < queryDesc->estate->es_range_table_size; i++) { ExecRowMark *thiserm = queryDesc->estate->es_rowmarks[i]; if (thiserm == NULL || !RowMarkRequiresRowShareLock(thiserm->markType)) continue; /* ignore non-FOR UPDATE/SHARE items */ if (thiserm->relid == table_oid) { if (erm) ereport(ERROR, (errcode(ERRCODE_INVALID_CURSOR_STATE), errmsg("cursor \"%s\" has multiple FOR UPDATE/SHARE references to table \"%s\"", cursor_name, table_name))); erm = thiserm; } } if (erm == NULL) ereport(ERROR, (errcode(ERRCODE_INVALID_CURSOR_STATE), errmsg("cursor \"%s\" does not have a FOR UPDATE/SHARE reference to table \"%s\"", cursor_name, table_name))); /* * The cursor must have a current result row: per the SQL spec, it's * an error if not. */ if (portal->atStart || portal->atEnd) ereport(ERROR, (errcode(ERRCODE_INVALID_CURSOR_STATE), errmsg("cursor \"%s\" is not positioned on a row", cursor_name))); /* Return the currently scanned TID, if there is one */ if (ItemPointerIsValid(&(erm->curCtid))) { *current_tid = erm->curCtid; return true; } /* * This table didn't produce the cursor's current row; some other * inheritance child of the same parent must have. Signal caller to * do nothing on this table. */ return false; } else { /* * Without FOR UPDATE, we dig through the cursor's plan to find the * scan node. Fail if it's not there or buried underneath * aggregation. */ ScanState *scanstate; bool pending_rescan = false; scanstate = search_plan_tree(queryDesc->planstate, table_oid, &pending_rescan); if (!scanstate) ereport(ERROR, (errcode(ERRCODE_INVALID_CURSOR_STATE), errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"", cursor_name, table_name))); /* * The cursor must have a current result row: per the SQL spec, it's * an error if not. We test this at the top level, rather than at the * scan node level, because in inheritance cases any one table scan * could easily not be on a row. We want to return false, not raise * error, if the passed-in table OID is for one of the inactive scans. */ if (portal->atStart || portal->atEnd) ereport(ERROR, (errcode(ERRCODE_INVALID_CURSOR_STATE), errmsg("cursor \"%s\" is not positioned on a row", cursor_name))); /* * Now OK to return false if we found an inactive scan. It is * inactive either if it's not positioned on a row, or there's a * rescan pending for it. */ if (TupIsNull(scanstate->ss_ScanTupleSlot) || pending_rescan) return false; /* * Extract TID of the scan's current row. The mechanism for this is * in principle scan-type-dependent, but for most scan types, we can * just dig the TID out of the physical scan tuple. */ if (IsA(scanstate, IndexOnlyScanState)) { /* * For IndexOnlyScan, the tuple stored in ss_ScanTupleSlot may be * a virtual tuple that does not have the ctid column, so we have * to get the TID from xs_ctup.t_self. */ IndexScanDesc scan = ((IndexOnlyScanState *) scanstate)->ioss_ScanDesc; *current_tid = scan->xs_heaptid; } else { /* * Default case: try to fetch TID from the scan node's current * tuple. As an extra cross-check, verify tableoid in the current * tuple. If the scan hasn't provided a physical tuple, we have * to fail. */ Datum ldatum; bool lisnull; ItemPointer tuple_tid; #ifdef USE_ASSERT_CHECKING ldatum = slot_getsysattr(scanstate->ss_ScanTupleSlot, TableOidAttributeNumber, &lisnull); if (lisnull) ereport(ERROR, (errcode(ERRCODE_INVALID_CURSOR_STATE), errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"", cursor_name, table_name))); Assert(DatumGetObjectId(ldatum) == table_oid); #endif ldatum = slot_getsysattr(scanstate->ss_ScanTupleSlot, SelfItemPointerAttributeNumber, &lisnull); if (lisnull) ereport(ERROR, (errcode(ERRCODE_INVALID_CURSOR_STATE), errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"", cursor_name, table_name))); tuple_tid = (ItemPointer) DatumGetPointer(ldatum); *current_tid = *tuple_tid; } Assert(ItemPointerIsValid(current_tid)); return true; } } /* * fetch_cursor_param_value * * Fetch the string value of a param, verifying it is of type REFCURSOR. */ static char * fetch_cursor_param_value(ExprContext *econtext, int paramId) { ParamListInfo paramInfo = econtext->ecxt_param_list_info; if (paramInfo && paramId > 0 && paramId <= paramInfo->numParams) { ParamExternData *prm; ParamExternData prmdata; /* give hook a chance in case parameter is dynamic */ if (paramInfo->paramFetch != NULL) prm = paramInfo->paramFetch(paramInfo, paramId, false, &prmdata); else prm = ¶mInfo->params[paramId - 1]; if (OidIsValid(prm->ptype) && !prm->isnull) { /* safety check in case hook did something unexpected */ if (prm->ptype != REFCURSOROID) ereport(ERROR, (errcode(ERRCODE_DATATYPE_MISMATCH), errmsg("type of parameter %d (%s) does not match that when preparing the plan (%s)", paramId, format_type_be(prm->ptype), format_type_be(REFCURSOROID)))); /* We know that refcursor uses text's I/O routines */ return TextDatumGetCString(prm->value); } } ereport(ERROR, (errcode(ERRCODE_UNDEFINED_OBJECT), errmsg("no value found for parameter %d", paramId))); return NULL; } /* * search_plan_tree * * Search through a PlanState tree for a scan node on the specified table. * Return NULL if not found or multiple candidates. * * CAUTION: this function is not charged simply with finding some candidate * scan, but with ensuring that that scan returned the plan tree's current * output row. That's why we must reject multiple-match cases. * * If a candidate is found, set *pending_rescan to true if that candidate * or any node above it has a pending rescan action, i.e. chgParam != NULL. * That indicates that we shouldn't consider the node to be positioned on a * valid tuple, even if its own state would indicate that it is. (Caller * must initialize *pending_rescan to false, and should not trust its state * if multiple candidates are found.) */ static ScanState * search_plan_tree(PlanState *node, Oid table_oid, bool *pending_rescan) { ScanState *result = NULL; if (node == NULL) return NULL; switch (nodeTag(node)) { /* * Relation scan nodes can all be treated alike: check to see if * they are scanning the specified table. * * ForeignScan and CustomScan might not have a currentRelation, in * which case we just ignore them. (We dare not descend to any * child plan nodes they might have, since we do not know the * relationship of such a node's current output tuple to the * children's current outputs.) */ case T_SeqScanState: case T_SampleScanState: case T_IndexScanState: case T_IndexOnlyScanState: case T_BitmapHeapScanState: case T_TidScanState: case T_TidRangeScanState: case T_ForeignScanState: case T_CustomScanState: { ScanState *sstate = (ScanState *) node; if (sstate->ss_currentRelation && RelationGetRelid(sstate->ss_currentRelation) == table_oid) result = sstate; break; } /* * For Append, we can check each input node. It is safe to * descend to the inputs because only the input that resulted in * the Append's current output node could be positioned on a tuple * at all; the other inputs are either at EOF or not yet started. * Hence, if the desired table is scanned by some * currently-inactive input node, we will find that node but then * our caller will realize that it didn't emit the tuple of * interest. * * We do need to watch out for multiple matches (possible if * Append was from UNION ALL rather than an inheritance tree). * * Note: we can NOT descend through MergeAppend similarly, since * its inputs are likely all active, and we don't know which one * returned the current output tuple. (Perhaps that could be * fixed if we were to let this code know more about MergeAppend's * internal state, but it does not seem worth the trouble. Users * should not expect plans for ORDER BY queries to be considered * simply-updatable, since they won't be if the sorting is * implemented by a Sort node.) */ case T_AppendState: { AppendState *astate = (AppendState *) node; int i; for (i = 0; i < astate->as_nplans; i++) { ScanState *elem = search_plan_tree(astate->appendplans[i], table_oid, pending_rescan); if (!elem) continue; if (result) return NULL; /* multiple matches */ result = elem; } break; } /* * Result and Limit can be descended through (these are safe * because they always return their input's current row) */ case T_ResultState: case T_LimitState: result = search_plan_tree(node->lefttree, table_oid, pending_rescan); break; /* * SubqueryScan too, but it keeps the child in a different place */ case T_SubqueryScanState: result = search_plan_tree(((SubqueryScanState *) node)->subplan, table_oid, pending_rescan); break; default: /* Otherwise, assume we can't descend through it */ break; } /* * If we found a candidate at or below this node, then this node's * chgParam indicates a pending rescan that will affect the candidate. */ if (result && node->chgParam != NULL) *pending_rescan = true; return result; }