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diff --git a/doc/src/sgml/html/index-unique-checks.html b/doc/src/sgml/html/index-unique-checks.html new file mode 100644 index 0000000..d136039 --- /dev/null +++ b/doc/src/sgml/html/index-unique-checks.html @@ -0,0 +1,109 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>64.5. Index Uniqueness Checks</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="index-locking.html" title="64.4. Index Locking Considerations" /><link rel="next" href="index-cost-estimation.html" title="64.6. Index Cost Estimation Functions" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">64.5. Index Uniqueness Checks</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="index-locking.html" title="64.4. Index Locking Considerations">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="indexam.html" title="Chapter 64. Index Access Method Interface Definition">Up</a></td><th width="60%" align="center">Chapter 64. Index Access Method Interface Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="index-cost-estimation.html" title="64.6. Index Cost Estimation Functions">Next</a></td></tr></table><hr /></div><div class="sect1" id="INDEX-UNIQUE-CHECKS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">64.5. Index Uniqueness Checks</h2></div></div></div><p> + <span class="productname">PostgreSQL</span> enforces SQL uniqueness constraints + using <em class="firstterm">unique indexes</em>, which are indexes that disallow + multiple entries with identical keys. An access method that supports this + feature sets <code class="structfield">amcanunique</code> true. + (At present, only b-tree supports it.) Columns listed in the + <code class="literal">INCLUDE</code> clause are not considered when enforcing + uniqueness. + </p><p> + Because of MVCC, it is always necessary to allow duplicate entries to + exist physically in an index: the entries might refer to successive + versions of a single logical row. The behavior we actually want to + enforce is that no MVCC snapshot could include two rows with equal + index keys. This breaks down into the following cases that must be + checked when inserting a new row into a unique index: + + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + If a conflicting valid row has been deleted by the current transaction, + it's okay. (In particular, since an UPDATE always deletes the old row + version before inserting the new version, this will allow an UPDATE on + a row without changing the key.) + </p></li><li class="listitem"><p> + If a conflicting row has been inserted by an as-yet-uncommitted + transaction, the would-be inserter must wait to see if that transaction + commits. If it rolls back then there is no conflict. If it commits + without deleting the conflicting row again, there is a uniqueness + violation. (In practice we just wait for the other transaction to + end and then redo the visibility check in toto.) + </p></li><li class="listitem"><p> + Similarly, if a conflicting valid row has been deleted by an + as-yet-uncommitted transaction, the would-be inserter must wait + for that transaction to commit or abort, and then repeat the test. + </p></li></ul></div><p> + </p><p> + Furthermore, immediately before reporting a uniqueness violation + according to the above rules, the access method must recheck the + liveness of the row being inserted. If it is committed dead then + no violation should be reported. (This case cannot occur during the + ordinary scenario of inserting a row that's just been created by + the current transaction. It can happen during + <code class="command">CREATE UNIQUE INDEX CONCURRENTLY</code>, however.) + </p><p> + We require the index access method to apply these tests itself, which + means that it must reach into the heap to check the commit status of + any row that is shown to have a duplicate key according to the index + contents. This is without a doubt ugly and non-modular, but it saves + redundant work: if we did a separate probe then the index lookup for + a conflicting row would be essentially repeated while finding the place to + insert the new row's index entry. What's more, there is no obvious way + to avoid race conditions unless the conflict check is an integral part + of insertion of the new index entry. + </p><p> + If the unique constraint is deferrable, there is additional complexity: + we need to be able to insert an index entry for a new row, but defer any + uniqueness-violation error until end of statement or even later. To + avoid unnecessary repeat searches of the index, the index access method + should do a preliminary uniqueness check during the initial insertion. + If this shows that there is definitely no conflicting live tuple, we + are done. Otherwise, we schedule a recheck to occur when it is time to + enforce the constraint. If, at the time of the recheck, both the inserted + tuple and some other tuple with the same key are live, then the error + must be reported. (Note that for this purpose, <span class="quote">“<span class="quote">live</span>”</span> actually + means <span class="quote">“<span class="quote">any tuple in the index entry's HOT chain is live</span>”</span>.) + To implement this, the <code class="function">aminsert</code> function is passed a + <code class="literal">checkUnique</code> parameter having one of the following values: + + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + <code class="literal">UNIQUE_CHECK_NO</code> indicates that no uniqueness checking + should be done (this is not a unique index). + </p></li><li class="listitem"><p> + <code class="literal">UNIQUE_CHECK_YES</code> indicates that this is a non-deferrable + unique index, and the uniqueness check must be done immediately, as + described above. + </p></li><li class="listitem"><p> + <code class="literal">UNIQUE_CHECK_PARTIAL</code> indicates that the unique + constraint is deferrable. <span class="productname">PostgreSQL</span> + will use this mode to insert each row's index entry. The access + method must allow duplicate entries into the index, and report any + potential duplicates by returning false from <code class="function">aminsert</code>. + For each row for which false is returned, a deferred recheck will + be scheduled. + </p><p> + The access method must identify any rows which might violate the + unique constraint, but it is not an error for it to report false + positives. This allows the check to be done without waiting for other + transactions to finish; conflicts reported here are not treated as + errors and will be rechecked later, by which time they may no longer + be conflicts. + </p></li><li class="listitem"><p> + <code class="literal">UNIQUE_CHECK_EXISTING</code> indicates that this is a deferred + recheck of a row that was reported as a potential uniqueness violation. + Although this is implemented by calling <code class="function">aminsert</code>, the + access method must <span class="emphasis"><em>not</em></span> insert a new index entry in this + case. The index entry is already present. Rather, the access method + must check to see if there is another live index entry. If so, and + if the target row is also still live, report error. + </p><p> + It is recommended that in a <code class="literal">UNIQUE_CHECK_EXISTING</code> call, + the access method further verify that the target row actually does + have an existing entry in the index, and report error if not. This + is a good idea because the index tuple values passed to + <code class="function">aminsert</code> will have been recomputed. If the index + definition involves functions that are not really immutable, we + might be checking the wrong area of the index. Checking that the + target row is found in the recheck verifies that we are scanning + for the same tuple values as were used in the original insertion. + </p></li></ul></div><p> + </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="index-locking.html" title="64.4. Index Locking Considerations">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="indexam.html" title="Chapter 64. Index Access Method Interface Definition">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="index-cost-estimation.html" title="64.6. Index Cost Estimation Functions">Next</a></td></tr><tr><td width="40%" align="left" valign="top">64.4. Index Locking Considerations </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 64.6. Index Cost Estimation Functions</td></tr></table></div></body></html>
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