diff options
Diffstat (limited to 'doc/src/sgml/html/index-scanning.html')
| -rw-r--r-- | doc/src/sgml/html/index-scanning.html | 123 |
1 files changed, 123 insertions, 0 deletions
diff --git a/doc/src/sgml/html/index-scanning.html b/doc/src/sgml/html/index-scanning.html new file mode 100644 index 0000000..0497418 --- /dev/null +++ b/doc/src/sgml/html/index-scanning.html @@ -0,0 +1,123 @@ +<?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>64.3. Index Scanning</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-functions.html" title="64.2. Index Access Method Functions" /><link rel="next" href="index-locking.html" title="64.4. Index Locking Considerations" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">64.3. Index Scanning</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="index-functions.html" title="64.2. Index Access Method Functions">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 15.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="index-locking.html" title="64.4. Index Locking Considerations">Next</a></td></tr></table><hr /></div><div class="sect1" id="INDEX-SCANNING"><div class="titlepage"><div><div><h2 class="title" style="clear: both">64.3. Index Scanning</h2></div></div></div><p> + In an index scan, the index access method is responsible for regurgitating + the TIDs of all the tuples it has been told about that match the + <em class="firstterm">scan keys</em>. The access method is <span class="emphasis"><em>not</em></span> involved in + actually fetching those tuples from the index's parent table, nor in + determining whether they pass the scan's visibility test or other + conditions. + </p><p> + A scan key is the internal representation of a <code class="literal">WHERE</code> clause of + the form <em class="replaceable"><code>index_key</code></em> <em class="replaceable"><code>operator</code></em> + <em class="replaceable"><code>constant</code></em>, where the index key is one of the columns of the + index and the operator is one of the members of the operator family + associated with that index column. An index scan has zero or more scan + keys, which are implicitly ANDed — the returned tuples are expected + to satisfy all the indicated conditions. + </p><p> + The access method can report that the index is <em class="firstterm">lossy</em>, or + requires rechecks, for a particular query. This implies that the index + scan will return all the entries that pass the scan key, plus possibly + additional entries that do not. The core system's index-scan machinery + will then apply the index conditions again to the heap tuple to verify + whether or not it really should be selected. If the recheck option is not + specified, the index scan must return exactly the set of matching entries. + </p><p> + Note that it is entirely up to the access method to ensure that it + correctly finds all and only the entries passing all the given scan keys. + Also, the core system will simply hand off all the <code class="literal">WHERE</code> + clauses that match the index keys and operator families, without any + semantic analysis to determine whether they are redundant or + contradictory. As an example, given + <code class="literal">WHERE x > 4 AND x > 14</code> where <code class="literal">x</code> is a b-tree + indexed column, it is left to the b-tree <code class="function">amrescan</code> function + to realize that the first scan key is redundant and can be discarded. + The extent of preprocessing needed during <code class="function">amrescan</code> will + depend on the extent to which the index access method needs to reduce + the scan keys to a <span class="quote">“<span class="quote">normalized</span>”</span> form. + </p><p> + Some access methods return index entries in a well-defined order, others + do not. There are actually two different ways that an access method can + support sorted output: + + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + Access methods that always return entries in the natural ordering + of their data (such as btree) should set + <code class="structfield">amcanorder</code> to true. + Currently, such access methods must use btree-compatible strategy + numbers for their equality and ordering operators. + </p></li><li class="listitem"><p> + Access methods that support ordering operators should set + <code class="structfield">amcanorderbyop</code> to true. + This indicates that the index is capable of returning entries in + an order satisfying <code class="literal">ORDER BY</code> <em class="replaceable"><code>index_key</code></em> + <em class="replaceable"><code>operator</code></em> <em class="replaceable"><code>constant</code></em>. Scan modifiers + of that form can be passed to <code class="function">amrescan</code> as described + previously. + </p></li></ul></div><p> + </p><p> + The <code class="function">amgettuple</code> function has a <code class="literal">direction</code> argument, + which can be either <code class="literal">ForwardScanDirection</code> (the normal case) + or <code class="literal">BackwardScanDirection</code>. If the first call after + <code class="function">amrescan</code> specifies <code class="literal">BackwardScanDirection</code>, then the + set of matching index entries is to be scanned back-to-front rather than in + the normal front-to-back direction, so <code class="function">amgettuple</code> must return + the last matching tuple in the index, rather than the first one as it + normally would. (This will only occur for access + methods that set <code class="structfield">amcanorder</code> to true.) After the + first call, <code class="function">amgettuple</code> must be prepared to advance the scan in + either direction from the most recently returned entry. (But if + <code class="structfield">amcanbackward</code> is false, all subsequent + calls will have the same direction as the first one.) + </p><p> + Access methods that support ordered scans must support <span class="quote">“<span class="quote">marking</span>”</span> a + position in a scan and later returning to the marked position. The same + position might be restored multiple times. However, only one position need + be remembered per scan; a new <code class="function">ammarkpos</code> call overrides the + previously marked position. An access method that does not support ordered + scans need not provide <code class="function">ammarkpos</code> and <code class="function">amrestrpos</code> + functions in <code class="structname">IndexAmRoutine</code>; set those pointers to NULL + instead. + </p><p> + Both the scan position and the mark position (if any) must be maintained + consistently in the face of concurrent insertions or deletions in the + index. It is OK if a freshly-inserted entry is not returned by a scan that + would have found the entry if it had existed when the scan started, or for + the scan to return such an entry upon rescanning or backing + up even though it had not been returned the first time through. Similarly, + a concurrent delete might or might not be reflected in the results of a scan. + What is important is that insertions or deletions not cause the scan to + miss or multiply return entries that were not themselves being inserted or + deleted. + </p><p> + If the index stores the original indexed data values (and not some lossy + representation of them), it is useful to + support <a class="link" href="indexes-index-only-scans.html" title="11.9. Index-Only Scans and Covering Indexes">index-only scans</a>, in + which the index returns the actual data not just the TID of the heap tuple. + This will only avoid I/O if the visibility map shows that the TID is on an + all-visible page; else the heap tuple must be visited anyway to check + MVCC visibility. But that is no concern of the access method's. + </p><p> + Instead of using <code class="function">amgettuple</code>, an index scan can be done with + <code class="function">amgetbitmap</code> to fetch all tuples in one call. This can be + noticeably more efficient than <code class="function">amgettuple</code> because it allows + avoiding lock/unlock cycles within the access method. In principle + <code class="function">amgetbitmap</code> should have the same effects as repeated + <code class="function">amgettuple</code> calls, but we impose several restrictions to + simplify matters. First of all, <code class="function">amgetbitmap</code> returns all + tuples at once and marking or restoring scan positions isn't + supported. Secondly, the tuples are returned in a bitmap which doesn't + have any specific ordering, which is why <code class="function">amgetbitmap</code> doesn't + take a <code class="literal">direction</code> argument. (Ordering operators will never be + supplied for such a scan, either.) + Also, there is no provision for index-only scans with + <code class="function">amgetbitmap</code>, since there is no way to return the contents of + index tuples. + Finally, <code class="function">amgetbitmap</code> + does not guarantee any locking of the returned tuples, with implications + spelled out in <a class="xref" href="index-locking.html" title="64.4. Index Locking Considerations">Section 64.4</a>. + </p><p> + Note that it is permitted for an access method to implement only + <code class="function">amgetbitmap</code> and not <code class="function">amgettuple</code>, or vice versa, + if its internal implementation is unsuited to one API or the other. + </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="index-functions.html" title="64.2. Index Access Method Functions">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-locking.html" title="64.4. Index Locking Considerations">Next</a></td></tr><tr><td width="40%" align="left" valign="top">64.2. Index Access Method Functions </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 64.4. Index Locking Considerations</td></tr></table></div></body></html>
\ No newline at end of file |