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diff --git a/doc/src/sgml/html/tablesample-support-functions.html b/doc/src/sgml/html/tablesample-support-functions.html new file mode 100644 index 0000000..efc28b2 --- /dev/null +++ b/doc/src/sgml/html/tablesample-support-functions.html @@ -0,0 +1,163 @@ +<?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>60.1. Sampling Method Support Functions</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="tablesample-method.html" title="Chapter 60. Writing a Table Sampling Method" /><link rel="next" href="custom-scan.html" title="Chapter 61. Writing a Custom Scan Provider" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">60.1. Sampling Method Support Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="tablesample-method.html" title="Chapter 60. Writing a Table Sampling Method">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="tablesample-method.html" title="Chapter 60. Writing a Table Sampling Method">Up</a></td><th width="60%" align="center">Chapter 60. Writing a Table Sampling Method</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="custom-scan.html" title="Chapter 61. Writing a Custom Scan Provider">Next</a></td></tr></table><hr /></div><div class="sect1" id="TABLESAMPLE-SUPPORT-FUNCTIONS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">60.1. Sampling Method Support Functions</h2></div></div></div><p> + The TSM handler function returns a palloc'd <code class="type">TsmRoutine</code> struct + containing pointers to the support functions described below. Most of + the functions are required, but some are optional, and those pointers can + be NULL. + </p><p> +</p><pre class="programlisting"> +void +SampleScanGetSampleSize (PlannerInfo *root, + RelOptInfo *baserel, + List *paramexprs, + BlockNumber *pages, + double *tuples); +</pre><p> + + This function is called during planning. It must estimate the number of + relation pages that will be read during a sample scan, and the number of + tuples that will be selected by the scan. (For example, these might be + determined by estimating the sampling fraction, and then multiplying + the <code class="literal">baserel->pages</code> and <code class="literal">baserel->tuples</code> + numbers by that, being sure to round the results to integral values.) + The <code class="literal">paramexprs</code> list holds the expression(s) that are + parameters to the <code class="literal">TABLESAMPLE</code> clause. It is recommended to + use <code class="function">estimate_expression_value()</code> to try to reduce these + expressions to constants, if their values are needed for estimation + purposes; but the function must provide size estimates even if they cannot + be reduced, and it should not fail even if the values appear invalid + (remember that they're only estimates of what the run-time values will be). + The <code class="literal">pages</code> and <code class="literal">tuples</code> parameters are outputs. + </p><p> +</p><pre class="programlisting"> +void +InitSampleScan (SampleScanState *node, + int eflags); +</pre><p> + + Initialize for execution of a SampleScan plan node. + This is called during executor startup. + It should perform any initialization needed before processing can start. + The <code class="structname">SampleScanState</code> node has already been created, but + its <code class="structfield">tsm_state</code> field is NULL. + The <code class="function">InitSampleScan</code> function can palloc whatever internal + state data is needed by the sampling method, and store a pointer to + it in <code class="literal">node->tsm_state</code>. + Information about the table to scan is accessible through other fields + of the <code class="structname">SampleScanState</code> node (but note that the + <code class="literal">node->ss.ss_currentScanDesc</code> scan descriptor is not set + up yet). + <code class="literal">eflags</code> contains flag bits describing the executor's + operating mode for this plan node. + </p><p> + When <code class="literal">(eflags & EXEC_FLAG_EXPLAIN_ONLY)</code> is true, + the scan will not actually be performed, so this function should only do + the minimum required to make the node state valid for <code class="command">EXPLAIN</code> + and <code class="function">EndSampleScan</code>. + </p><p> + This function can be omitted (set the pointer to NULL), in which case + <code class="function">BeginSampleScan</code> must perform all initialization needed + by the sampling method. + </p><p> +</p><pre class="programlisting"> +void +BeginSampleScan (SampleScanState *node, + Datum *params, + int nparams, + uint32 seed); +</pre><p> + + Begin execution of a sampling scan. + This is called just before the first attempt to fetch a tuple, and + may be called again if the scan needs to be restarted. + Information about the table to scan is accessible through fields + of the <code class="structname">SampleScanState</code> node (but note that the + <code class="literal">node->ss.ss_currentScanDesc</code> scan descriptor is not set + up yet). + The <code class="literal">params</code> array, of length <code class="literal">nparams</code>, contains the + values of the parameters supplied in the <code class="literal">TABLESAMPLE</code> clause. + These will have the number and types specified in the sampling + method's <code class="literal">parameterTypes</code> list, and have been checked + to not be null. + <code class="literal">seed</code> contains a seed to use for any random numbers generated + within the sampling method; it is either a hash derived from the + <code class="literal">REPEATABLE</code> value if one was given, or the result + of <code class="literal">random()</code> if not. + </p><p> + This function may adjust the fields <code class="literal">node->use_bulkread</code> + and <code class="literal">node->use_pagemode</code>. + If <code class="literal">node->use_bulkread</code> is <code class="literal">true</code>, which it is by + default, the scan will use a buffer access strategy that encourages + recycling buffers after use. It might be reasonable to set this + to <code class="literal">false</code> if the scan will visit only a small fraction of the + table's pages. + If <code class="literal">node->use_pagemode</code> is <code class="literal">true</code>, which it is by + default, the scan will perform visibility checking in a single pass for + all tuples on each visited page. It might be reasonable to set this + to <code class="literal">false</code> if the scan will select only a small fraction of the + tuples on each visited page. That will result in fewer tuple visibility + checks being performed, though each one will be more expensive because it + will require more locking. + </p><p> + If the sampling method is + marked <code class="literal">repeatable_across_scans</code>, it must be able to + select the same set of tuples during a rescan as it did originally, that is + a fresh call of <code class="function">BeginSampleScan</code> must lead to selecting the + same tuples as before (if the <code class="literal">TABLESAMPLE</code> parameters + and seed don't change). + </p><p> +</p><pre class="programlisting"> +BlockNumber +NextSampleBlock (SampleScanState *node, BlockNumber nblocks); +</pre><p> + + Returns the block number of the next page to be scanned, or + <code class="literal">InvalidBlockNumber</code> if no pages remain to be scanned. + </p><p> + This function can be omitted (set the pointer to NULL), in which case + the core code will perform a sequential scan of the entire relation. + Such a scan can use synchronized scanning, so that the sampling method + cannot assume that the relation pages are visited in the same order on + each scan. + </p><p> +</p><pre class="programlisting"> +OffsetNumber +NextSampleTuple (SampleScanState *node, + BlockNumber blockno, + OffsetNumber maxoffset); +</pre><p> + + Returns the offset number of the next tuple to be sampled on the + specified page, or <code class="literal">InvalidOffsetNumber</code> if no tuples remain to + be sampled. <code class="literal">maxoffset</code> is the largest offset number in use + on the page. + </p><div class="note"><h3 class="title">Note</h3><p> + <code class="function">NextSampleTuple</code> is not explicitly told which of the offset + numbers in the range <code class="literal">1 .. maxoffset</code> actually contain valid + tuples. This is not normally a problem since the core code ignores + requests to sample missing or invisible tuples; that should not result in + any bias in the sample. However, if necessary, the function can use + <code class="literal">node->donetuples</code> to examine how many of the tuples + it returned were valid and visible. + </p></div><div class="note"><h3 class="title">Note</h3><p> + <code class="function">NextSampleTuple</code> must <span class="emphasis"><em>not</em></span> assume + that <code class="literal">blockno</code> is the same page number returned by the most + recent <code class="function">NextSampleBlock</code> call. It was returned by some + previous <code class="function">NextSampleBlock</code> call, but the core code is allowed + to call <code class="function">NextSampleBlock</code> in advance of actually scanning + pages, so as to support prefetching. It is OK to assume that once + sampling of a given page begins, successive <code class="function">NextSampleTuple</code> + calls all refer to the same page until <code class="literal">InvalidOffsetNumber</code> is + returned. + </p></div><p> +</p><pre class="programlisting"> +void +EndSampleScan (SampleScanState *node); +</pre><p> + + End the scan and release resources. It is normally not important + to release palloc'd memory, but any externally-visible resources + should be cleaned up. + This function can be omitted (set the pointer to NULL) in the common + case where no such resources exist. + </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="tablesample-method.html" title="Chapter 60. Writing a Table Sampling Method">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="tablesample-method.html" title="Chapter 60. 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