Line data Source code
1 : /*-------------------------------------------------------------------------
2 : *
3 : * toast_internals.c
4 : * Functions for internal use by the TOAST system.
5 : *
6 : * Copyright (c) 2000-2024, PostgreSQL Global Development Group
7 : *
8 : * IDENTIFICATION
9 : * src/backend/access/common/toast_internals.c
10 : *
11 : *-------------------------------------------------------------------------
12 : */
13 :
14 : #include "postgres.h"
15 :
16 : #include "access/detoast.h"
17 : #include "access/genam.h"
18 : #include "access/heapam.h"
19 : #include "access/heaptoast.h"
20 : #include "access/table.h"
21 : #include "access/toast_internals.h"
22 : #include "access/xact.h"
23 : #include "catalog/catalog.h"
24 : #include "miscadmin.h"
25 : #include "utils/fmgroids.h"
26 : #include "utils/rel.h"
27 : #include "utils/snapmgr.h"
28 :
29 : static bool toastrel_valueid_exists(Relation toastrel, Oid valueid);
30 : static bool toastid_valueid_exists(Oid toastrelid, Oid valueid);
31 :
32 : /* ----------
33 : * toast_compress_datum -
34 : *
35 : * Create a compressed version of a varlena datum
36 : *
37 : * If we fail (ie, compressed result is actually bigger than original)
38 : * then return NULL. We must not use compressed data if it'd expand
39 : * the tuple!
40 : *
41 : * We use VAR{SIZE,DATA}_ANY so we can handle short varlenas here without
42 : * copying them. But we can't handle external or compressed datums.
43 : * ----------
44 : */
45 : Datum
46 44102 : toast_compress_datum(Datum value, char cmethod)
47 : {
48 44102 : struct varlena *tmp = NULL;
49 : int32 valsize;
50 44102 : ToastCompressionId cmid = TOAST_INVALID_COMPRESSION_ID;
51 :
52 : Assert(!VARATT_IS_EXTERNAL(DatumGetPointer(value)));
53 : Assert(!VARATT_IS_COMPRESSED(DatumGetPointer(value)));
54 :
55 44102 : valsize = VARSIZE_ANY_EXHDR(DatumGetPointer(value));
56 :
57 : /* If the compression method is not valid, use the current default */
58 44102 : if (!CompressionMethodIsValid(cmethod))
59 44030 : cmethod = default_toast_compression;
60 :
61 : /*
62 : * Call appropriate compression routine for the compression method.
63 : */
64 44102 : switch (cmethod)
65 : {
66 44066 : case TOAST_PGLZ_COMPRESSION:
67 44066 : tmp = pglz_compress_datum((const struct varlena *) value);
68 44066 : cmid = TOAST_PGLZ_COMPRESSION_ID;
69 44066 : break;
70 36 : case TOAST_LZ4_COMPRESSION:
71 36 : tmp = lz4_compress_datum((const struct varlena *) value);
72 36 : cmid = TOAST_LZ4_COMPRESSION_ID;
73 36 : break;
74 0 : default:
75 0 : elog(ERROR, "invalid compression method %c", cmethod);
76 : }
77 :
78 44102 : if (tmp == NULL)
79 11734 : return PointerGetDatum(NULL);
80 :
81 : /*
82 : * We recheck the actual size even if compression reports success, because
83 : * it might be satisfied with having saved as little as one byte in the
84 : * compressed data --- which could turn into a net loss once you consider
85 : * header and alignment padding. Worst case, the compressed format might
86 : * require three padding bytes (plus header, which is included in
87 : * VARSIZE(tmp)), whereas the uncompressed format would take only one
88 : * header byte and no padding if the value is short enough. So we insist
89 : * on a savings of more than 2 bytes to ensure we have a gain.
90 : */
91 32368 : if (VARSIZE(tmp) < valsize - 2)
92 : {
93 : /* successful compression */
94 : Assert(cmid != TOAST_INVALID_COMPRESSION_ID);
95 32368 : TOAST_COMPRESS_SET_SIZE_AND_COMPRESS_METHOD(tmp, valsize, cmid);
96 32368 : return PointerGetDatum(tmp);
97 : }
98 : else
99 : {
100 : /* incompressible data */
101 0 : pfree(tmp);
102 0 : return PointerGetDatum(NULL);
103 : }
104 : }
105 :
106 : /* ----------
107 : * toast_save_datum -
108 : *
109 : * Save one single datum into the secondary relation and return
110 : * a Datum reference for it.
111 : *
112 : * rel: the main relation we're working with (not the toast rel!)
113 : * value: datum to be pushed to toast storage
114 : * oldexternal: if not NULL, toast pointer previously representing the datum
115 : * options: options to be passed to heap_insert() for toast rows
116 : * ----------
117 : */
118 : Datum
119 14192 : toast_save_datum(Relation rel, Datum value,
120 : struct varlena *oldexternal, int options)
121 : {
122 : Relation toastrel;
123 : Relation *toastidxs;
124 : HeapTuple toasttup;
125 : TupleDesc toasttupDesc;
126 : Datum t_values[3];
127 : bool t_isnull[3];
128 14192 : CommandId mycid = GetCurrentCommandId(true);
129 : struct varlena *result;
130 : struct varatt_external toast_pointer;
131 : union
132 : {
133 : struct varlena hdr;
134 : /* this is to make the union big enough for a chunk: */
135 : char data[TOAST_MAX_CHUNK_SIZE + VARHDRSZ];
136 : /* ensure union is aligned well enough: */
137 : int32 align_it;
138 : } chunk_data;
139 : int32 chunk_size;
140 14192 : int32 chunk_seq = 0;
141 : char *data_p;
142 : int32 data_todo;
143 14192 : Pointer dval = DatumGetPointer(value);
144 : int num_indexes;
145 : int validIndex;
146 :
147 : Assert(!VARATT_IS_EXTERNAL(value));
148 :
149 : /*
150 : * Open the toast relation and its indexes. We can use the index to check
151 : * uniqueness of the OID we assign to the toasted item, even though it has
152 : * additional columns besides OID.
153 : */
154 14192 : toastrel = table_open(rel->rd_rel->reltoastrelid, RowExclusiveLock);
155 14192 : toasttupDesc = toastrel->rd_att;
156 :
157 : /* Open all the toast indexes and look for the valid one */
158 14192 : validIndex = toast_open_indexes(toastrel,
159 : RowExclusiveLock,
160 : &toastidxs,
161 : &num_indexes);
162 :
163 : /*
164 : * Get the data pointer and length, and compute va_rawsize and va_extinfo.
165 : *
166 : * va_rawsize is the size of the equivalent fully uncompressed datum, so
167 : * we have to adjust for short headers.
168 : *
169 : * va_extinfo stored the actual size of the data payload in the toast
170 : * records and the compression method in first 2 bits if data is
171 : * compressed.
172 : */
173 14192 : if (VARATT_IS_SHORT(dval))
174 : {
175 0 : data_p = VARDATA_SHORT(dval);
176 0 : data_todo = VARSIZE_SHORT(dval) - VARHDRSZ_SHORT;
177 0 : toast_pointer.va_rawsize = data_todo + VARHDRSZ; /* as if not short */
178 0 : toast_pointer.va_extinfo = data_todo;
179 : }
180 14192 : else if (VARATT_IS_COMPRESSED(dval))
181 : {
182 7986 : data_p = VARDATA(dval);
183 7986 : data_todo = VARSIZE(dval) - VARHDRSZ;
184 : /* rawsize in a compressed datum is just the size of the payload */
185 7986 : toast_pointer.va_rawsize = VARDATA_COMPRESSED_GET_EXTSIZE(dval) + VARHDRSZ;
186 :
187 : /* set external size and compression method */
188 7986 : VARATT_EXTERNAL_SET_SIZE_AND_COMPRESS_METHOD(toast_pointer, data_todo,
189 : VARDATA_COMPRESSED_GET_COMPRESS_METHOD(dval));
190 : /* Assert that the numbers look like it's compressed */
191 : Assert(VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer));
192 : }
193 : else
194 : {
195 6206 : data_p = VARDATA(dval);
196 6206 : data_todo = VARSIZE(dval) - VARHDRSZ;
197 6206 : toast_pointer.va_rawsize = VARSIZE(dval);
198 6206 : toast_pointer.va_extinfo = data_todo;
199 : }
200 :
201 : /*
202 : * Insert the correct table OID into the result TOAST pointer.
203 : *
204 : * Normally this is the actual OID of the target toast table, but during
205 : * table-rewriting operations such as CLUSTER, we have to insert the OID
206 : * of the table's real permanent toast table instead. rd_toastoid is set
207 : * if we have to substitute such an OID.
208 : */
209 14192 : if (OidIsValid(rel->rd_toastoid))
210 580 : toast_pointer.va_toastrelid = rel->rd_toastoid;
211 : else
212 13612 : toast_pointer.va_toastrelid = RelationGetRelid(toastrel);
213 :
214 : /*
215 : * Choose an OID to use as the value ID for this toast value.
216 : *
217 : * Normally we just choose an unused OID within the toast table. But
218 : * during table-rewriting operations where we are preserving an existing
219 : * toast table OID, we want to preserve toast value OIDs too. So, if
220 : * rd_toastoid is set and we had a prior external value from that same
221 : * toast table, re-use its value ID. If we didn't have a prior external
222 : * value (which is a corner case, but possible if the table's attstorage
223 : * options have been changed), we have to pick a value ID that doesn't
224 : * conflict with either new or existing toast value OIDs.
225 : */
226 14192 : if (!OidIsValid(rel->rd_toastoid))
227 : {
228 : /* normal case: just choose an unused OID */
229 13612 : toast_pointer.va_valueid =
230 13612 : GetNewOidWithIndex(toastrel,
231 13612 : RelationGetRelid(toastidxs[validIndex]),
232 : (AttrNumber) 1);
233 : }
234 : else
235 : {
236 : /* rewrite case: check to see if value was in old toast table */
237 580 : toast_pointer.va_valueid = InvalidOid;
238 580 : if (oldexternal != NULL)
239 : {
240 : struct varatt_external old_toast_pointer;
241 :
242 : Assert(VARATT_IS_EXTERNAL_ONDISK(oldexternal));
243 : /* Must copy to access aligned fields */
244 580 : VARATT_EXTERNAL_GET_POINTER(old_toast_pointer, oldexternal);
245 580 : if (old_toast_pointer.va_toastrelid == rel->rd_toastoid)
246 : {
247 : /* This value came from the old toast table; reuse its OID */
248 580 : toast_pointer.va_valueid = old_toast_pointer.va_valueid;
249 :
250 : /*
251 : * There is a corner case here: the table rewrite might have
252 : * to copy both live and recently-dead versions of a row, and
253 : * those versions could easily reference the same toast value.
254 : * When we copy the second or later version of such a row,
255 : * reusing the OID will mean we select an OID that's already
256 : * in the new toast table. Check for that, and if so, just
257 : * fall through without writing the data again.
258 : *
259 : * While annoying and ugly-looking, this is a good thing
260 : * because it ensures that we wind up with only one copy of
261 : * the toast value when there is only one copy in the old
262 : * toast table. Before we detected this case, we'd have made
263 : * multiple copies, wasting space; and what's worse, the
264 : * copies belonging to already-deleted heap tuples would not
265 : * be reclaimed by VACUUM.
266 : */
267 580 : if (toastrel_valueid_exists(toastrel,
268 : toast_pointer.va_valueid))
269 : {
270 : /* Match, so short-circuit the data storage loop below */
271 0 : data_todo = 0;
272 : }
273 : }
274 : }
275 580 : if (toast_pointer.va_valueid == InvalidOid)
276 : {
277 : /*
278 : * new value; must choose an OID that doesn't conflict in either
279 : * old or new toast table
280 : */
281 : do
282 : {
283 0 : toast_pointer.va_valueid =
284 0 : GetNewOidWithIndex(toastrel,
285 0 : RelationGetRelid(toastidxs[validIndex]),
286 : (AttrNumber) 1);
287 0 : } while (toastid_valueid_exists(rel->rd_toastoid,
288 0 : toast_pointer.va_valueid));
289 : }
290 : }
291 :
292 : /*
293 : * Initialize constant parts of the tuple data
294 : */
295 14192 : t_values[0] = ObjectIdGetDatum(toast_pointer.va_valueid);
296 14192 : t_values[2] = PointerGetDatum(&chunk_data);
297 14192 : t_isnull[0] = false;
298 14192 : t_isnull[1] = false;
299 14192 : t_isnull[2] = false;
300 :
301 : /*
302 : * Split up the item into chunks
303 : */
304 66078 : while (data_todo > 0)
305 : {
306 : int i;
307 :
308 51886 : CHECK_FOR_INTERRUPTS();
309 :
310 : /*
311 : * Calculate the size of this chunk
312 : */
313 51886 : chunk_size = Min(TOAST_MAX_CHUNK_SIZE, data_todo);
314 :
315 : /*
316 : * Build a tuple and store it
317 : */
318 51886 : t_values[1] = Int32GetDatum(chunk_seq++);
319 51886 : SET_VARSIZE(&chunk_data, chunk_size + VARHDRSZ);
320 51886 : memcpy(VARDATA(&chunk_data), data_p, chunk_size);
321 51886 : toasttup = heap_form_tuple(toasttupDesc, t_values, t_isnull);
322 :
323 51886 : heap_insert(toastrel, toasttup, mycid, options, NULL);
324 :
325 : /*
326 : * Create the index entry. We cheat a little here by not using
327 : * FormIndexDatum: this relies on the knowledge that the index columns
328 : * are the same as the initial columns of the table for all the
329 : * indexes. We also cheat by not providing an IndexInfo: this is okay
330 : * for now because btree doesn't need one, but we might have to be
331 : * more honest someday.
332 : *
333 : * Note also that there had better not be any user-created index on
334 : * the TOAST table, since we don't bother to update anything else.
335 : */
336 103772 : for (i = 0; i < num_indexes; i++)
337 : {
338 : /* Only index relations marked as ready can be updated */
339 51886 : if (toastidxs[i]->rd_index->indisready)
340 51886 : index_insert(toastidxs[i], t_values, t_isnull,
341 : &(toasttup->t_self),
342 : toastrel,
343 51886 : toastidxs[i]->rd_index->indisunique ?
344 : UNIQUE_CHECK_YES : UNIQUE_CHECK_NO,
345 : false, NULL);
346 : }
347 :
348 : /*
349 : * Free memory
350 : */
351 51886 : heap_freetuple(toasttup);
352 :
353 : /*
354 : * Move on to next chunk
355 : */
356 51886 : data_todo -= chunk_size;
357 51886 : data_p += chunk_size;
358 : }
359 :
360 : /*
361 : * Done - close toast relation and its indexes but keep the lock until
362 : * commit, so as a concurrent reindex done directly on the toast relation
363 : * would be able to wait for this transaction.
364 : */
365 14192 : toast_close_indexes(toastidxs, num_indexes, NoLock);
366 14192 : table_close(toastrel, NoLock);
367 :
368 : /*
369 : * Create the TOAST pointer value that we'll return
370 : */
371 14192 : result = (struct varlena *) palloc(TOAST_POINTER_SIZE);
372 14192 : SET_VARTAG_EXTERNAL(result, VARTAG_ONDISK);
373 14192 : memcpy(VARDATA_EXTERNAL(result), &toast_pointer, sizeof(toast_pointer));
374 :
375 14192 : return PointerGetDatum(result);
376 : }
377 :
378 : /* ----------
379 : * toast_delete_datum -
380 : *
381 : * Delete a single external stored value.
382 : * ----------
383 : */
384 : void
385 996 : toast_delete_datum(Relation rel, Datum value, bool is_speculative)
386 : {
387 996 : struct varlena *attr = (struct varlena *) DatumGetPointer(value);
388 : struct varatt_external toast_pointer;
389 : Relation toastrel;
390 : Relation *toastidxs;
391 : ScanKeyData toastkey;
392 : SysScanDesc toastscan;
393 : HeapTuple toasttup;
394 : int num_indexes;
395 : int validIndex;
396 : SnapshotData SnapshotToast;
397 :
398 996 : if (!VARATT_IS_EXTERNAL_ONDISK(attr))
399 0 : return;
400 :
401 : /* Must copy to access aligned fields */
402 996 : VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr);
403 :
404 : /*
405 : * Open the toast relation and its indexes
406 : */
407 996 : toastrel = table_open(toast_pointer.va_toastrelid, RowExclusiveLock);
408 :
409 : /* Fetch valid relation used for process */
410 996 : validIndex = toast_open_indexes(toastrel,
411 : RowExclusiveLock,
412 : &toastidxs,
413 : &num_indexes);
414 :
415 : /*
416 : * Setup a scan key to find chunks with matching va_valueid
417 : */
418 996 : ScanKeyInit(&toastkey,
419 : (AttrNumber) 1,
420 : BTEqualStrategyNumber, F_OIDEQ,
421 : ObjectIdGetDatum(toast_pointer.va_valueid));
422 :
423 : /*
424 : * Find all the chunks. (We don't actually care whether we see them in
425 : * sequence or not, but since we've already locked the index we might as
426 : * well use systable_beginscan_ordered.)
427 : */
428 996 : init_toast_snapshot(&SnapshotToast);
429 996 : toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
430 : &SnapshotToast, 1, &toastkey);
431 4284 : while ((toasttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
432 : {
433 : /*
434 : * Have a chunk, delete it
435 : */
436 3288 : if (is_speculative)
437 10 : heap_abort_speculative(toastrel, &toasttup->t_self);
438 : else
439 3278 : simple_heap_delete(toastrel, &toasttup->t_self);
440 : }
441 :
442 : /*
443 : * End scan and close relations but keep the lock until commit, so as a
444 : * concurrent reindex done directly on the toast relation would be able to
445 : * wait for this transaction.
446 : */
447 996 : systable_endscan_ordered(toastscan);
448 996 : toast_close_indexes(toastidxs, num_indexes, NoLock);
449 996 : table_close(toastrel, NoLock);
450 : }
451 :
452 : /* ----------
453 : * toastrel_valueid_exists -
454 : *
455 : * Test whether a toast value with the given ID exists in the toast relation.
456 : * For safety, we consider a value to exist if there are either live or dead
457 : * toast rows with that ID; see notes for GetNewOidWithIndex().
458 : * ----------
459 : */
460 : static bool
461 580 : toastrel_valueid_exists(Relation toastrel, Oid valueid)
462 : {
463 580 : bool result = false;
464 : ScanKeyData toastkey;
465 : SysScanDesc toastscan;
466 : int num_indexes;
467 : int validIndex;
468 : Relation *toastidxs;
469 :
470 : /* Fetch a valid index relation */
471 580 : validIndex = toast_open_indexes(toastrel,
472 : RowExclusiveLock,
473 : &toastidxs,
474 : &num_indexes);
475 :
476 : /*
477 : * Setup a scan key to find chunks with matching va_valueid
478 : */
479 580 : ScanKeyInit(&toastkey,
480 : (AttrNumber) 1,
481 : BTEqualStrategyNumber, F_OIDEQ,
482 : ObjectIdGetDatum(valueid));
483 :
484 : /*
485 : * Is there any such chunk?
486 : */
487 580 : toastscan = systable_beginscan(toastrel,
488 580 : RelationGetRelid(toastidxs[validIndex]),
489 : true, SnapshotAny, 1, &toastkey);
490 :
491 580 : if (systable_getnext(toastscan) != NULL)
492 0 : result = true;
493 :
494 580 : systable_endscan(toastscan);
495 :
496 : /* Clean up */
497 580 : toast_close_indexes(toastidxs, num_indexes, RowExclusiveLock);
498 :
499 580 : return result;
500 : }
501 :
502 : /* ----------
503 : * toastid_valueid_exists -
504 : *
505 : * As above, but work from toast rel's OID not an open relation
506 : * ----------
507 : */
508 : static bool
509 0 : toastid_valueid_exists(Oid toastrelid, Oid valueid)
510 : {
511 : bool result;
512 : Relation toastrel;
513 :
514 0 : toastrel = table_open(toastrelid, AccessShareLock);
515 :
516 0 : result = toastrel_valueid_exists(toastrel, valueid);
517 :
518 0 : table_close(toastrel, AccessShareLock);
519 :
520 0 : return result;
521 : }
522 :
523 : /* ----------
524 : * toast_get_valid_index
525 : *
526 : * Get OID of valid index associated to given toast relation. A toast
527 : * relation can have only one valid index at the same time.
528 : */
529 : Oid
530 768 : toast_get_valid_index(Oid toastoid, LOCKMODE lock)
531 : {
532 : int num_indexes;
533 : int validIndex;
534 : Oid validIndexOid;
535 : Relation *toastidxs;
536 : Relation toastrel;
537 :
538 : /* Open the toast relation */
539 768 : toastrel = table_open(toastoid, lock);
540 :
541 : /* Look for the valid index of the toast relation */
542 768 : validIndex = toast_open_indexes(toastrel,
543 : lock,
544 : &toastidxs,
545 : &num_indexes);
546 768 : validIndexOid = RelationGetRelid(toastidxs[validIndex]);
547 :
548 : /* Close the toast relation and all its indexes */
549 768 : toast_close_indexes(toastidxs, num_indexes, NoLock);
550 768 : table_close(toastrel, NoLock);
551 :
552 768 : return validIndexOid;
553 : }
554 :
555 : /* ----------
556 : * toast_open_indexes
557 : *
558 : * Get an array of the indexes associated to the given toast relation
559 : * and return as well the position of the valid index used by the toast
560 : * relation in this array. It is the responsibility of the caller of this
561 : * function to close the indexes as well as free them.
562 : */
563 : int
564 35628 : toast_open_indexes(Relation toastrel,
565 : LOCKMODE lock,
566 : Relation **toastidxs,
567 : int *num_indexes)
568 : {
569 35628 : int i = 0;
570 35628 : int res = 0;
571 35628 : bool found = false;
572 : List *indexlist;
573 : ListCell *lc;
574 :
575 : /* Get index list of the toast relation */
576 35628 : indexlist = RelationGetIndexList(toastrel);
577 : Assert(indexlist != NIL);
578 :
579 35628 : *num_indexes = list_length(indexlist);
580 :
581 : /* Open all the index relations */
582 35628 : *toastidxs = (Relation *) palloc(*num_indexes * sizeof(Relation));
583 71256 : foreach(lc, indexlist)
584 35628 : (*toastidxs)[i++] = index_open(lfirst_oid(lc), lock);
585 :
586 : /* Fetch the first valid index in list */
587 35628 : for (i = 0; i < *num_indexes; i++)
588 : {
589 35628 : Relation toastidx = (*toastidxs)[i];
590 :
591 35628 : if (toastidx->rd_index->indisvalid)
592 : {
593 35628 : res = i;
594 35628 : found = true;
595 35628 : break;
596 : }
597 : }
598 :
599 : /*
600 : * Free index list, not necessary anymore as relations are opened and a
601 : * valid index has been found.
602 : */
603 35628 : list_free(indexlist);
604 :
605 : /*
606 : * The toast relation should have one valid index, so something is going
607 : * wrong if there is nothing.
608 : */
609 35628 : if (!found)
610 0 : elog(ERROR, "no valid index found for toast relation with Oid %u",
611 : RelationGetRelid(toastrel));
612 :
613 35628 : return res;
614 : }
615 :
616 : /* ----------
617 : * toast_close_indexes
618 : *
619 : * Close an array of indexes for a toast relation and free it. This should
620 : * be called for a set of indexes opened previously with toast_open_indexes.
621 : */
622 : void
623 35622 : toast_close_indexes(Relation *toastidxs, int num_indexes, LOCKMODE lock)
624 : {
625 : int i;
626 :
627 : /* Close relations and clean up things */
628 71244 : for (i = 0; i < num_indexes; i++)
629 35622 : index_close(toastidxs[i], lock);
630 35622 : pfree(toastidxs);
631 35622 : }
632 :
633 : /* ----------
634 : * init_toast_snapshot
635 : *
636 : * Initialize an appropriate TOAST snapshot. We must use an MVCC snapshot
637 : * to initialize the TOAST snapshot; since we don't know which one to use,
638 : * just use the oldest one.
639 : */
640 : void
641 42326 : init_toast_snapshot(Snapshot toast_snapshot)
642 : {
643 42326 : Snapshot snapshot = GetOldestSnapshot();
644 :
645 : /*
646 : * GetOldestSnapshot returns NULL if the session has no active snapshots.
647 : * We can get that if, for example, a procedure fetches a toasted value
648 : * into a local variable, commits, and then tries to detoast the value.
649 : * Such coding is unsafe, because once we commit there is nothing to
650 : * prevent the toast data from being deleted. Detoasting *must* happen in
651 : * the same transaction that originally fetched the toast pointer. Hence,
652 : * rather than trying to band-aid over the problem, throw an error. (This
653 : * is not very much protection, because in many scenarios the procedure
654 : * would have already created a new transaction snapshot, preventing us
655 : * from detecting the problem. But it's better than nothing, and for sure
656 : * we shouldn't expend code on masking the problem more.)
657 : */
658 42326 : if (snapshot == NULL)
659 0 : elog(ERROR, "cannot fetch toast data without an active snapshot");
660 :
661 : /*
662 : * Catalog snapshots can be returned by GetOldestSnapshot() even if not
663 : * registered or active. That easily hides bugs around not having a
664 : * snapshot set up - most of the time there is a valid catalog snapshot.
665 : * So additionally insist that the current snapshot is registered or
666 : * active.
667 : */
668 : Assert(HaveRegisteredOrActiveSnapshot());
669 :
670 42326 : InitToastSnapshot(*toast_snapshot, snapshot->lsn, snapshot->whenTaken);
671 42326 : }
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