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
26static char *fetch_cursor_param_value(ExprContext *econtext, int paramId);
27static 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 */
43bool
44execCurrentOf(CurrentOfExpr *cexpr,
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 ||
110 !RowMarkRequiresRowShareLock(thiserm->markType))
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,
223 TableOidAttributeNumber,
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,
234 SelfItemPointerAttributeNumber,
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 */
257static char *
258fetch_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 */
309static ScanState *
310search_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:
325 case T_IndexOnlyScanState:
326 case T_BitmapHeapScanState:
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}
419