Line data Source code
1 : /*-------------------------------------------------------------------------
2 : *
3 : * combocid.c
4 : * Combo command ID support routines
5 : *
6 : * Before version 8.3, HeapTupleHeaderData had separate fields for cmin
7 : * and cmax. To reduce the header size, cmin and cmax are now overlaid
8 : * in the same field in the header. That usually works because you rarely
9 : * insert and delete a tuple in the same transaction, and we don't need
10 : * either field to remain valid after the originating transaction exits.
11 : * To make it work when the inserting transaction does delete the tuple,
12 : * we create a "combo" command ID and store that in the tuple header
13 : * instead of cmin and cmax. The combo command ID can be mapped to the
14 : * real cmin and cmax using a backend-private array, which is managed by
15 : * this module.
16 : *
17 : * To allow reusing existing combo CIDs, we also keep a hash table that
18 : * maps cmin,cmax pairs to combo CIDs. This keeps the data structure size
19 : * reasonable in most cases, since the number of unique pairs used by any
20 : * one transaction is likely to be small.
21 : *
22 : * With a 32-bit combo command id we can represent 2^32 distinct cmin,cmax
23 : * combinations. In the most perverse case where each command deletes a tuple
24 : * generated by every previous command, the number of combo command ids
25 : * required for N commands is N*(N+1)/2. That means that in the worst case,
26 : * that's enough for 92682 commands. In practice, you'll run out of memory
27 : * and/or disk space way before you reach that limit.
28 : *
29 : * The array and hash table are kept in TopTransactionContext, and are
30 : * destroyed at the end of each transaction.
31 : *
32 : *
33 : * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
34 : * Portions Copyright (c) 1994, Regents of the University of California
35 : *
36 : * IDENTIFICATION
37 : * src/backend/utils/time/combocid.c
38 : *
39 : *-------------------------------------------------------------------------
40 : */
41 :
42 : #include "postgres.h"
43 :
44 : #include "access/htup_details.h"
45 : #include "access/xact.h"
46 : #include "miscadmin.h"
47 : #include "storage/shmem.h"
48 : #include "utils/combocid.h"
49 : #include "utils/hsearch.h"
50 : #include "utils/memutils.h"
51 :
52 : /* Hash table to lookup combo CIDs by cmin and cmax */
53 : static HTAB *comboHash = NULL;
54 :
55 : /* Key and entry structures for the hash table */
56 : typedef struct
57 : {
58 : CommandId cmin;
59 : CommandId cmax;
60 : } ComboCidKeyData;
61 :
62 : typedef ComboCidKeyData *ComboCidKey;
63 :
64 : typedef struct
65 : {
66 : ComboCidKeyData key;
67 : CommandId combocid;
68 : } ComboCidEntryData;
69 :
70 : typedef ComboCidEntryData *ComboCidEntry;
71 :
72 : /* Initial size of the hash table */
73 : #define CCID_HASH_SIZE 100
74 :
75 :
76 : /*
77 : * An array of cmin,cmax pairs, indexed by combo command id.
78 : * To convert a combo CID to cmin and cmax, you do a simple array lookup.
79 : */
80 : static ComboCidKey comboCids = NULL;
81 : static int usedComboCids = 0; /* number of elements in comboCids */
82 : static int sizeComboCids = 0; /* allocated size of array */
83 :
84 : /* Initial size of the array */
85 : #define CCID_ARRAY_SIZE 100
86 :
87 :
88 : /* prototypes for internal functions */
89 : static CommandId GetComboCommandId(CommandId cmin, CommandId cmax);
90 : static CommandId GetRealCmin(CommandId combocid);
91 : static CommandId GetRealCmax(CommandId combocid);
92 :
93 :
94 : /**** External API ****/
95 :
96 : /*
97 : * GetCmin and GetCmax assert that they are only called in situations where
98 : * they make sense, that is, can deliver a useful answer. If you have
99 : * reason to examine a tuple's t_cid field from a transaction other than
100 : * the originating one, use HeapTupleHeaderGetRawCommandId() directly.
101 : */
102 :
103 : CommandId
104 11435770 : HeapTupleHeaderGetCmin(const HeapTupleHeaderData *tup)
105 : {
106 11435770 : CommandId cid = HeapTupleHeaderGetRawCommandId(tup);
107 :
108 : Assert(!(tup->t_infomask & HEAP_MOVED));
109 : Assert(TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetXmin(tup)));
110 :
111 11435770 : if (tup->t_infomask & HEAP_COMBOCID)
112 2968544 : return GetRealCmin(cid);
113 : else
114 8467226 : return cid;
115 : }
116 :
117 : CommandId
118 3113252 : HeapTupleHeaderGetCmax(const HeapTupleHeaderData *tup)
119 : {
120 3113252 : CommandId cid = HeapTupleHeaderGetRawCommandId(tup);
121 :
122 : Assert(!(tup->t_infomask & HEAP_MOVED));
123 :
124 : /*
125 : * Because GetUpdateXid() performs memory allocations if xmax is a
126 : * multixact we can't Assert() if we're inside a critical section. This
127 : * weakens the check, but not using GetCmax() inside one would complicate
128 : * things too much.
129 : */
130 : Assert(CritSectionCount > 0 ||
131 : TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetUpdateXid(tup)));
132 :
133 3113252 : if (tup->t_infomask & HEAP_COMBOCID)
134 2968378 : return GetRealCmax(cid);
135 : else
136 144874 : return cid;
137 : }
138 :
139 : /*
140 : * Given a tuple we are about to delete, determine the correct value to store
141 : * into its t_cid field.
142 : *
143 : * If we don't need a combo CID, *cmax is unchanged and *iscombo is set to
144 : * false. If we do need one, *cmax is replaced by a combo CID and *iscombo
145 : * is set to true.
146 : *
147 : * The reason this is separate from the actual HeapTupleHeaderSetCmax()
148 : * operation is that this could fail due to out-of-memory conditions. Hence
149 : * we need to do this before entering the critical section that actually
150 : * changes the tuple in shared buffers.
151 : */
152 : void
153 1884943 : HeapTupleHeaderAdjustCmax(const HeapTupleHeaderData *tup,
154 : CommandId *cmax,
155 : bool *iscombo)
156 : {
157 : /*
158 : * If we're marking a tuple deleted that was inserted by (any
159 : * subtransaction of) our transaction, we need to use a combo command id.
160 : * Test for HeapTupleHeaderXminCommitted() first, because it's cheaper
161 : * than a TransactionIdIsCurrentTransactionId call.
162 : */
163 2104331 : if (!HeapTupleHeaderXminCommitted(tup) &&
164 219388 : TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmin(tup)))
165 218227 : {
166 218227 : CommandId cmin = HeapTupleHeaderGetCmin(tup);
167 :
168 218227 : *cmax = GetComboCommandId(cmin, *cmax);
169 218227 : *iscombo = true;
170 : }
171 : else
172 : {
173 1666716 : *iscombo = false;
174 : }
175 1884943 : }
176 :
177 : /*
178 : * Combo command ids are only interesting to the inserting and deleting
179 : * transaction, so we can forget about them at the end of transaction.
180 : */
181 : void
182 564875 : AtEOXact_ComboCid(void)
183 : {
184 : /*
185 : * Don't bother to pfree. These are allocated in TopTransactionContext, so
186 : * they're going to go away at the end of transaction anyway.
187 : */
188 564875 : comboHash = NULL;
189 :
190 564875 : comboCids = NULL;
191 564875 : usedComboCids = 0;
192 564875 : sizeComboCids = 0;
193 564875 : }
194 :
195 :
196 : /**** Internal routines ****/
197 :
198 : /*
199 : * Get a combo command id that maps to cmin and cmax.
200 : *
201 : * We try to reuse old combo command ids when possible.
202 : */
203 : static CommandId
204 246673 : GetComboCommandId(CommandId cmin, CommandId cmax)
205 : {
206 : CommandId combocid;
207 : ComboCidKeyData key;
208 : ComboCidEntry entry;
209 : bool found;
210 :
211 : /*
212 : * Create the hash table and array the first time we need to use combo
213 : * cids in the transaction.
214 : */
215 246673 : if (comboHash == NULL)
216 : {
217 : HASHCTL hash_ctl;
218 :
219 : /* Make array first; existence of hash table asserts array exists */
220 23742 : comboCids = (ComboCidKeyData *)
221 23742 : MemoryContextAlloc(TopTransactionContext,
222 : sizeof(ComboCidKeyData) * CCID_ARRAY_SIZE);
223 23742 : sizeComboCids = CCID_ARRAY_SIZE;
224 23742 : usedComboCids = 0;
225 :
226 23742 : hash_ctl.keysize = sizeof(ComboCidKeyData);
227 23742 : hash_ctl.entrysize = sizeof(ComboCidEntryData);
228 23742 : hash_ctl.hcxt = TopTransactionContext;
229 :
230 23742 : comboHash = hash_create("Combo CIDs",
231 : CCID_HASH_SIZE,
232 : &hash_ctl,
233 : HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
234 : }
235 :
236 : /*
237 : * Grow the array if there's not at least one free slot. We must do this
238 : * before possibly entering a new hashtable entry, else failure to
239 : * repalloc would leave a corrupt hashtable entry behind.
240 : */
241 246673 : if (usedComboCids >= sizeComboCids)
242 : {
243 81 : int newsize = sizeComboCids * 2;
244 :
245 81 : comboCids = (ComboCidKeyData *)
246 81 : repalloc(comboCids, sizeof(ComboCidKeyData) * newsize);
247 81 : sizeComboCids = newsize;
248 : }
249 :
250 : /* Lookup or create a hash entry with the desired cmin/cmax */
251 :
252 : /* We assume there is no struct padding in ComboCidKeyData! */
253 246673 : key.cmin = cmin;
254 246673 : key.cmax = cmax;
255 246673 : entry = (ComboCidEntry) hash_search(comboHash,
256 : &key,
257 : HASH_ENTER,
258 : &found);
259 :
260 246673 : if (found)
261 : {
262 : /* Reuse an existing combo CID */
263 125650 : return entry->combocid;
264 : }
265 :
266 : /* We have to create a new combo CID; we already made room in the array */
267 121023 : combocid = usedComboCids;
268 :
269 121023 : comboCids[combocid].cmin = cmin;
270 121023 : comboCids[combocid].cmax = cmax;
271 121023 : usedComboCids++;
272 :
273 121023 : entry->combocid = combocid;
274 :
275 121023 : return combocid;
276 : }
277 :
278 : static CommandId
279 2968544 : GetRealCmin(CommandId combocid)
280 : {
281 : Assert(combocid < usedComboCids);
282 2968544 : return comboCids[combocid].cmin;
283 : }
284 :
285 : static CommandId
286 2968378 : GetRealCmax(CommandId combocid)
287 : {
288 : Assert(combocid < usedComboCids);
289 2968378 : return comboCids[combocid].cmax;
290 : }
291 :
292 : /*
293 : * Estimate the amount of space required to serialize the current combo CID
294 : * state.
295 : */
296 : Size
297 498 : EstimateComboCIDStateSpace(void)
298 : {
299 : Size size;
300 :
301 : /* Add space required for saving usedComboCids */
302 498 : size = sizeof(int);
303 :
304 : /* Add space required for saving ComboCidKeyData */
305 498 : size = add_size(size, mul_size(sizeof(ComboCidKeyData), usedComboCids));
306 :
307 498 : return size;
308 : }
309 :
310 : /*
311 : * Serialize the combo CID state into the memory, beginning at start_address.
312 : * maxsize should be at least as large as the value returned by
313 : * EstimateComboCIDStateSpace.
314 : */
315 : void
316 498 : SerializeComboCIDState(Size maxsize, char *start_address)
317 : {
318 : char *endptr;
319 :
320 : /* First, we store the number of currently-existing combo CIDs. */
321 498 : *(int *) start_address = usedComboCids;
322 :
323 : /* If maxsize is too small, throw an error. */
324 498 : endptr = start_address + sizeof(int) +
325 498 : (sizeof(ComboCidKeyData) * usedComboCids);
326 498 : if (endptr < start_address || endptr > start_address + maxsize)
327 0 : elog(ERROR, "not enough space to serialize ComboCID state");
328 :
329 : /* Now, copy the actual cmin/cmax pairs. */
330 498 : if (usedComboCids > 0)
331 257 : memcpy(start_address + sizeof(int), comboCids,
332 : (sizeof(ComboCidKeyData) * usedComboCids));
333 498 : }
334 :
335 : /*
336 : * Read the combo CID state at the specified address and initialize this
337 : * backend with the same combo CIDs. This is only valid in a backend that
338 : * currently has no combo CIDs (and only makes sense if the transaction state
339 : * is serialized and restored as well).
340 : */
341 : void
342 1489 : RestoreComboCIDState(char *comboCIDstate)
343 : {
344 : int num_elements;
345 : ComboCidKeyData *keydata;
346 : int i;
347 : CommandId cid;
348 :
349 : Assert(!comboCids && !comboHash);
350 :
351 : /* First, we retrieve the number of combo CIDs that were serialized. */
352 1489 : num_elements = *(int *) comboCIDstate;
353 1489 : keydata = (ComboCidKeyData *) (comboCIDstate + sizeof(int));
354 :
355 : /* Use GetComboCommandId to restore each combo CID. */
356 29935 : for (i = 0; i < num_elements; i++)
357 : {
358 28446 : cid = GetComboCommandId(keydata[i].cmin, keydata[i].cmax);
359 :
360 : /* Verify that we got the expected answer. */
361 28446 : if (cid != i)
362 0 : elog(ERROR, "unexpected command ID while restoring combo CIDs");
363 : }
364 1489 : }
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