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1 | 1 | /*------------------------------------------------------------------------- |
2 | 2 | * |
3 | 3 | * execIndexing.c |
4 | | - * executor support for maintaining indexes |
| 4 | + * routines for inserting index tuples and enforcing unique and |
| 5 | + * exclusive constraints. |
| 6 | + * |
| 7 | + * ExecInsertIndexTuples() is the main entry point. It's called after |
| 8 | + * inserting a tuple to the heap, and it inserts corresponding index tuples |
| 9 | + * into all indexes. At the same time, it enforces any unique and |
| 10 | + * exclusion constraints: |
| 11 | + * |
| 12 | + * Unique Indexes |
| 13 | + * -------------- |
| 14 | + * |
| 15 | + * Enforcing a unique constraint is straightforward. When the index AM |
| 16 | + * inserts the tuple to the index, it also checks that there are no |
| 17 | + * conflicting tuples in the index already. It does so atomically, so that |
| 18 | + * even if two backends try to insert the same key concurrently, only one |
| 19 | + * of them will succeed. All the logic to ensure atomicity, and to wait |
| 20 | + * for in-progress transactions to finish, is handled by the index AM. |
| 21 | + * |
| 22 | + * If a unique constraint is deferred, we request the index AM to not |
| 23 | + * throw an error if a conflict is found. Instead, we make note that there |
| 24 | + * was a conflict and return the list of indexes with conflicts to the |
| 25 | + * caller. The caller must re-check them later, by calling index_insert() |
| 26 | + * with the UNIQUE_CHECK_EXISTING option. |
| 27 | + * |
| 28 | + * Exclusion Constraints |
| 29 | + * --------------------- |
| 30 | + * |
| 31 | + * Exclusion constraints are different from unique indexes in that when the |
| 32 | + * tuple is inserted to the index, the index AM does not check for |
| 33 | + * duplicate keys at the same time. After the insertion, we perform a |
| 34 | + * separate scan on the index to check for conflicting tuples, and if one |
| 35 | + * is found, we throw an error and the transaction is aborted. If the |
| 36 | + * conflicting tuple's inserter or deleter is in-progress, we wait for it |
| 37 | + * to finish first. |
| 38 | + * |
| 39 | + * There is a chance of deadlock, if two backends insert a tuple at the |
| 40 | + * same time, and then perform the scan to check for conflicts. They will |
| 41 | + * find each other's tuple, and both try to wait for each other. The |
| 42 | + * deadlock detector will detect that, and abort one of the transactions. |
| 43 | + * That's fairly harmless, as one of them was bound to abort with a |
| 44 | + * "duplicate key error" anyway, although you get a different error |
| 45 | + * message. |
| 46 | + * |
| 47 | + * If an exclusion constraint is deferred, we still perform the conflict |
| 48 | + * checking scan immediately after inserting the index tuple. But instead |
| 49 | + * of throwing an error if a conflict is found, we return that information |
| 50 | + * to the caller. The caller must re-check them later by calling |
| 51 | + * check_exclusion_constraint(). |
| 52 | + * |
5 | 53 | * |
6 | 54 | * Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group |
7 | 55 | * Portions Copyright (c) 1994, Regents of the University of California |
@@ -134,13 +182,10 @@ ExecCloseIndices(ResultRelInfo *resultRelInfo) |
134 | 182 | *This routine takes care of inserting index tuples |
135 | 183 | *into all the relations indexing the result relation |
136 | 184 | *when a heap tuple is inserted into the result relation. |
137 | | - *Much of this code should be moved into the genam |
138 | | - *stuff as it only exists here because the genam stuff |
139 | | - *doesn't provide the functionality needed by the |
140 | | - *executor.. -cim 9/27/89 |
141 | 185 | * |
142 | | - *This returns a list of index OIDs for any unique or exclusion |
143 | | - *constraints that are deferred and that had |
| 186 | + *Unique and exclusion constraints are enforced at the same |
| 187 | + *time. This returns a list of index OIDs for any unique or |
| 188 | + *exclusion constraints that are deferred and that had |
144 | 189 | *potential (unconfirmed) conflicts. |
145 | 190 | * |
146 | 191 | *CAUTION: this must not be called for a HOT update. |
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