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execCurrent.c
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1 /*-------------------------------------------------------------------------
2  *
3  * execCurrent.c
4  * executor support for WHERE CURRENT OF cursor
5  *
6  * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
7  * Portions Copyright (c) 1994, Regents of the University of California
8  *
9  * src/backend/executor/execCurrent.c
10  *
11  *-------------------------------------------------------------------------
12  */
13 #include "postgres.h"
14 
15 #include "access/genam.h"
16 #include "access/relscan.h"
17 #include "access/sysattr.h"
18 #include "catalog/pg_type.h"
19 #include "executor/executor.h"
20 #include "utils/builtins.h"
21 #include "utils/lsyscache.h"
22 #include "utils/portal.h"
23 #include "utils/rel.h"
24 
25 
26 static char *fetch_cursor_param_value(ExprContext *econtext, int paramId);
27 static ScanState *search_plan_tree(PlanState *node, Oid table_oid,
28  bool *pending_rescan);
29 
30 
31 /*
32  * execCurrentOf
33  *
34  * Given a CURRENT OF expression and the OID of a table, determine which row
35  * of the table is currently being scanned by the cursor named by CURRENT OF,
36  * and return the row's TID into *current_tid.
37  *
38  * Returns true if a row was identified. Returns false if the cursor is valid
39  * for the table but is not currently scanning a row of the table (this is a
40  * legal situation in inheritance cases). Raises error if cursor is not a
41  * valid updatable scan of the specified table.
42  */
43 bool
45  ExprContext *econtext,
46  Oid table_oid,
47  ItemPointer current_tid)
48 {
49  char *cursor_name;
50  char *table_name;
51  Portal portal;
52  QueryDesc *queryDesc;
53 
54  /* Get the cursor name --- may have to look up a parameter reference */
55  if (cexpr->cursor_name)
56  cursor_name = cexpr->cursor_name;
57  else
58  cursor_name = fetch_cursor_param_value(econtext, cexpr->cursor_param);
59 
60  /* Fetch table name for possible use in error messages */
61  table_name = get_rel_name(table_oid);
62  if (table_name == NULL)
63  elog(ERROR, "cache lookup failed for relation %u", table_oid);
64 
65  /* Find the cursor's portal */
66  portal = GetPortalByName(cursor_name);
67  if (!PortalIsValid(portal))
68  ereport(ERROR,
69  (errcode(ERRCODE_UNDEFINED_CURSOR),
70  errmsg("cursor \"%s\" does not exist", cursor_name)));
71 
72  /*
73  * We have to watch out for non-SELECT queries as well as held cursors,
74  * both of which may have null queryDesc.
75  */
76  if (portal->strategy != PORTAL_ONE_SELECT)
77  ereport(ERROR,
78  (errcode(ERRCODE_INVALID_CURSOR_STATE),
79  errmsg("cursor \"%s\" is not a SELECT query",
80  cursor_name)));
81  queryDesc = portal->queryDesc;
82  if (queryDesc == NULL || queryDesc->estate == NULL)
83  ereport(ERROR,
84  (errcode(ERRCODE_INVALID_CURSOR_STATE),
85  errmsg("cursor \"%s\" is held from a previous transaction",
86  cursor_name)));
87 
88  /*
89  * We have two different strategies depending on whether the cursor uses
90  * FOR UPDATE/SHARE or not. The reason for supporting both is that the
91  * FOR UPDATE code is able to identify a target table in many cases where
92  * the other code can't, while the non-FOR-UPDATE case allows use of WHERE
93  * CURRENT OF with an insensitive cursor.
94  */
95  if (queryDesc->estate->es_rowmarks)
96  {
97  ExecRowMark *erm;
98  Index i;
99 
100  /*
101  * Here, the query must have exactly one FOR UPDATE/SHARE reference to
102  * the target table, and we dig the ctid info out of that.
103  */
104  erm = NULL;
105  for (i = 0; i < queryDesc->estate->es_range_table_size; i++)
106  {
107  ExecRowMark *thiserm = queryDesc->estate->es_rowmarks[i];
108 
109  if (thiserm == NULL ||
111  continue; /* ignore non-FOR UPDATE/SHARE items */
112 
113  if (thiserm->relid == table_oid)
114  {
115  if (erm)
116  ereport(ERROR,
117  (errcode(ERRCODE_INVALID_CURSOR_STATE),
118  errmsg("cursor \"%s\" has multiple FOR UPDATE/SHARE references to table \"%s\"",
119  cursor_name, table_name)));
120  erm = thiserm;
121  }
122  }
123 
124  if (erm == NULL)
125  ereport(ERROR,
126  (errcode(ERRCODE_INVALID_CURSOR_STATE),
127  errmsg("cursor \"%s\" does not have a FOR UPDATE/SHARE reference to table \"%s\"",
128  cursor_name, table_name)));
129 
130  /*
131  * The cursor must have a current result row: per the SQL spec, it's
132  * an error if not.
133  */
134  if (portal->atStart || portal->atEnd)
135  ereport(ERROR,
136  (errcode(ERRCODE_INVALID_CURSOR_STATE),
137  errmsg("cursor \"%s\" is not positioned on a row",
138  cursor_name)));
139 
140  /* Return the currently scanned TID, if there is one */
141  if (ItemPointerIsValid(&(erm->curCtid)))
142  {
143  *current_tid = erm->curCtid;
144  return true;
145  }
146 
147  /*
148  * This table didn't produce the cursor's current row; some other
149  * inheritance child of the same parent must have. Signal caller to
150  * do nothing on this table.
151  */
152  return false;
153  }
154  else
155  {
156  /*
157  * Without FOR UPDATE, we dig through the cursor's plan to find the
158  * scan node. Fail if it's not there or buried underneath
159  * aggregation.
160  */
161  ScanState *scanstate;
162  bool pending_rescan = false;
163 
164  scanstate = search_plan_tree(queryDesc->planstate, table_oid,
165  &pending_rescan);
166  if (!scanstate)
167  ereport(ERROR,
168  (errcode(ERRCODE_INVALID_CURSOR_STATE),
169  errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"",
170  cursor_name, table_name)));
171 
172  /*
173  * The cursor must have a current result row: per the SQL spec, it's
174  * an error if not. We test this at the top level, rather than at the
175  * scan node level, because in inheritance cases any one table scan
176  * could easily not be on a row. We want to return false, not raise
177  * error, if the passed-in table OID is for one of the inactive scans.
178  */
179  if (portal->atStart || portal->atEnd)
180  ereport(ERROR,
181  (errcode(ERRCODE_INVALID_CURSOR_STATE),
182  errmsg("cursor \"%s\" is not positioned on a row",
183  cursor_name)));
184 
185  /*
186  * Now OK to return false if we found an inactive scan. It is
187  * inactive either if it's not positioned on a row, or there's a
188  * rescan pending for it.
189  */
190  if (TupIsNull(scanstate->ss_ScanTupleSlot) || pending_rescan)
191  return false;
192 
193  /*
194  * Extract TID of the scan's current row. The mechanism for this is
195  * in principle scan-type-dependent, but for most scan types, we can
196  * just dig the TID out of the physical scan tuple.
197  */
198  if (IsA(scanstate, IndexOnlyScanState))
199  {
200  /*
201  * For IndexOnlyScan, the tuple stored in ss_ScanTupleSlot may be
202  * a virtual tuple that does not have the ctid column, so we have
203  * to get the TID from xs_ctup.t_self.
204  */
205  IndexScanDesc scan = ((IndexOnlyScanState *) scanstate)->ioss_ScanDesc;
206 
207  *current_tid = scan->xs_heaptid;
208  }
209  else
210  {
211  /*
212  * Default case: try to fetch TID from the scan node's current
213  * tuple. As an extra cross-check, verify tableoid in the current
214  * tuple. If the scan hasn't provided a physical tuple, we have
215  * to fail.
216  */
217  Datum ldatum;
218  bool lisnull;
219  ItemPointer tuple_tid;
220 
221 #ifdef USE_ASSERT_CHECKING
222  ldatum = slot_getsysattr(scanstate->ss_ScanTupleSlot,
224  &lisnull);
225  if (lisnull)
226  ereport(ERROR,
227  (errcode(ERRCODE_INVALID_CURSOR_STATE),
228  errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"",
229  cursor_name, table_name)));
230  Assert(DatumGetObjectId(ldatum) == table_oid);
231 #endif
232 
233  ldatum = slot_getsysattr(scanstate->ss_ScanTupleSlot,
235  &lisnull);
236  if (lisnull)
237  ereport(ERROR,
238  (errcode(ERRCODE_INVALID_CURSOR_STATE),
239  errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"",
240  cursor_name, table_name)));
241  tuple_tid = (ItemPointer) DatumGetPointer(ldatum);
242 
243  *current_tid = *tuple_tid;
244  }
245 
246  Assert(ItemPointerIsValid(current_tid));
247 
248  return true;
249  }
250 }
251 
252 /*
253  * fetch_cursor_param_value
254  *
255  * Fetch the string value of a param, verifying it is of type REFCURSOR.
256  */
257 static char *
258 fetch_cursor_param_value(ExprContext *econtext, int paramId)
259 {
260  ParamListInfo paramInfo = econtext->ecxt_param_list_info;
261 
262  if (paramInfo &&
263  paramId > 0 && paramId <= paramInfo->numParams)
264  {
265  ParamExternData *prm;
266  ParamExternData prmdata;
267 
268  /* give hook a chance in case parameter is dynamic */
269  if (paramInfo->paramFetch != NULL)
270  prm = paramInfo->paramFetch(paramInfo, paramId, false, &prmdata);
271  else
272  prm = &paramInfo->params[paramId - 1];
273 
274  if (OidIsValid(prm->ptype) && !prm->isnull)
275  {
276  /* safety check in case hook did something unexpected */
277  if (prm->ptype != REFCURSOROID)
278  ereport(ERROR,
279  (errcode(ERRCODE_DATATYPE_MISMATCH),
280  errmsg("type of parameter %d (%s) does not match that when preparing the plan (%s)",
281  paramId,
282  format_type_be(prm->ptype),
283  format_type_be(REFCURSOROID))));
284 
285  /* We know that refcursor uses text's I/O routines */
286  return TextDatumGetCString(prm->value);
287  }
288  }
289 
290  ereport(ERROR,
291  (errcode(ERRCODE_UNDEFINED_OBJECT),
292  errmsg("no value found for parameter %d", paramId)));
293  return NULL;
294 }
295 
296 /*
297  * search_plan_tree
298  *
299  * Search through a PlanState tree for a scan node on the specified table.
300  * Return NULL if not found or multiple candidates.
301  *
302  * If a candidate is found, set *pending_rescan to true if that candidate
303  * or any node above it has a pending rescan action, i.e. chgParam != NULL.
304  * That indicates that we shouldn't consider the node to be positioned on a
305  * valid tuple, even if its own state would indicate that it is. (Caller
306  * must initialize *pending_rescan to false, and should not trust its state
307  * if multiple candidates are found.)
308  */
309 static ScanState *
310 search_plan_tree(PlanState *node, Oid table_oid,
311  bool *pending_rescan)
312 {
313  ScanState *result = NULL;
314 
315  if (node == NULL)
316  return NULL;
317  switch (nodeTag(node))
318  {
319  /*
320  * Relation scan nodes can all be treated alike
321  */
322  case T_SeqScanState:
323  case T_SampleScanState:
324  case T_IndexScanState:
327  case T_TidScanState:
328  case T_ForeignScanState:
329  case T_CustomScanState:
330  {
331  ScanState *sstate = (ScanState *) node;
332 
333  if (RelationGetRelid(sstate->ss_currentRelation) == table_oid)
334  result = sstate;
335  break;
336  }
337 
338  /*
339  * For Append, we must look through the members; watch out for
340  * multiple matches (possible if it was from UNION ALL)
341  */
342  case T_AppendState:
343  {
344  AppendState *astate = (AppendState *) node;
345  int i;
346 
347  for (i = 0; i < astate->as_nplans; i++)
348  {
349  ScanState *elem = search_plan_tree(astate->appendplans[i],
350  table_oid,
351  pending_rescan);
352 
353  if (!elem)
354  continue;
355  if (result)
356  return NULL; /* multiple matches */
357  result = elem;
358  }
359  break;
360  }
361 
362  /*
363  * Similarly for MergeAppend
364  */
365  case T_MergeAppendState:
366  {
367  MergeAppendState *mstate = (MergeAppendState *) node;
368  int i;
369 
370  for (i = 0; i < mstate->ms_nplans; i++)
371  {
372  ScanState *elem = search_plan_tree(mstate->mergeplans[i],
373  table_oid,
374  pending_rescan);
375 
376  if (!elem)
377  continue;
378  if (result)
379  return NULL; /* multiple matches */
380  result = elem;
381  }
382  break;
383  }
384 
385  /*
386  * Result and Limit can be descended through (these are safe
387  * because they always return their input's current row)
388  */
389  case T_ResultState:
390  case T_LimitState:
391  result = search_plan_tree(node->lefttree,
392  table_oid,
393  pending_rescan);
394  break;
395 
396  /*
397  * SubqueryScan too, but it keeps the child in a different place
398  */
399  case T_SubqueryScanState:
400  result = search_plan_tree(((SubqueryScanState *) node)->subplan,
401  table_oid,
402  pending_rescan);
403  break;
404 
405  default:
406  /* Otherwise, assume we can't descend through it */
407  break;
408  }
409 
410  /*
411  * If we found a candidate at or below this node, then this node's
412  * chgParam indicates a pending rescan that will affect the candidate.
413  */
414  if (result && node->chgParam != NULL)
415  *pending_rescan = true;
416 
417  return result;
418 }
#define ItemPointerIsValid(pointer)
Definition: itemptr.h:82
ParamExternData params[FLEXIBLE_ARRAY_MEMBER]
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#define IsA(nodeptr, _type_)
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