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The original implementation of TABLESAMPLE modeled the tablesample method API on index access methods, which wasn't a good choice because, without specialized DDL commands, there's no way to build an extension that can implement a TSM. (Raw inserts into system catalogs are not an acceptable thing to do, because we can't undo them during DROP EXTENSION, nor will pg_upgrade behave sanely.) Instead adopt an API more like procedural language handlers or foreign data wrappers, wherein the only SQL-level support object needed is a single handler function identified by having a special return type. This lets us get rid of the supporting catalog altogether, so that no custom DDL support is needed for the feature. Adjust the API so that it can support non-constant tablesample arguments (the original coding assumed we could evaluate the argument expressions at ExecInitSampleScan time, which is undesirable even if it weren't outright unsafe), and discourage sampling methods from looking at invisible tuples. Make sure that the BERNOULLI and SYSTEM methods are genuinely repeatable within and across queries, as required by the SQL standard, and deal more honestly with methods that can't support that requirement. Make a full code-review pass over the tablesample additions, and fix assorted bugs, omissions, infelicities, and cosmetic issues (such as failure to put the added code stanzas in a consistent ordering). Improve EXPLAIN's output of tablesample plans, too. Back-patch to 9.5 so that we don't have to support the original API in production.
302 lines
12 KiB
Plaintext
302 lines
12 KiB
Plaintext
<!-- doc/src/sgml/tablesample-method.sgml -->
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<chapter id="tablesample-method">
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<title>Writing A Table Sampling Method</title>
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<indexterm zone="tablesample-method">
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<primary>table sampling method</primary>
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</indexterm>
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<indexterm zone="tablesample-method">
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<primary><literal>TABLESAMPLE</literal> method</primary>
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</indexterm>
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<para>
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<productname>PostgreSQL</>'s implementation of the <literal>TABLESAMPLE</>
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clause supports custom table sampling methods, in addition to
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the <literal>BERNOULLI</> and <literal>SYSTEM</> methods that are required
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by the SQL standard. The sampling method determines which rows of the
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table will be selected when the <literal>TABLESAMPLE</> clause is used.
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</para>
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<para>
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At the SQL level, a table sampling method is represented by a single SQL
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function, typically implemented in C, having the signature
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<programlisting>
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method_name(internal) RETURNS tsm_handler
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</programlisting>
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The name of the function is the same method name appearing in the
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<literal>TABLESAMPLE</> clause. The <type>internal</> argument is a dummy
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(always having value zero) that simply serves to prevent this function from
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being called directly from a SQL command.
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The result of the function must be a palloc'd struct of
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type <type>TsmRoutine</>, which contains pointers to support functions for
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the sampling method. These support functions are plain C functions and
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are not visible or callable at the SQL level. The support functions are
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described in <xref linkend="tablesample-support-functions">.
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</para>
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<para>
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In addition to function pointers, the <type>TsmRoutine</> struct must
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provide these additional fields:
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>List *parameterTypes</literal></term>
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<listitem>
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<para>
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This is an OID list containing the data type OIDs of the parameter(s)
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that will be accepted by the <literal>TABLESAMPLE</> clause when this
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sampling method is used. For example, for the built-in methods, this
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list contains a single item with value <literal>FLOAT4OID</>, which
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represents the sampling percentage. Custom sampling methods can have
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more or different parameters.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>bool repeatable_across_queries</literal></term>
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<listitem>
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<para>
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If <literal>true</>, the sampling method can deliver identical samples
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across successive queries, if the same parameters
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and <literal>REPEATABLE</> seed value are supplied each time and the
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table contents have not changed. When this is <literal>false</>,
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the <literal>REPEATABLE</> clause is not accepted for use with the
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sampling method.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>bool repeatable_across_scans</literal></term>
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<listitem>
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<para>
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If <literal>true</>, the sampling method can deliver identical samples
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across successive scans in the same query (assuming unchanging
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parameters, seed value, and snapshot).
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When this is <literal>false</>, the planner will not select plans that
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would require scanning the sampled table more than once, since that
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might result in inconsistent query output.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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The <type>TsmRoutine</> struct type is declared
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in <filename>src/include/access/tsmapi.h</>, which see for additional
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details.
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</para>
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<para>
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The table sampling methods included in the standard distribution are good
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references when trying to write your own. Look into
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the <filename>src/backend/access/tablesample</> subdirectory of the source
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tree for the built-in sampling methods, and into the <filename>contrib</>
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subdirectory for add-on methods.
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</para>
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<sect1 id="tablesample-support-functions">
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<title>Sampling Method Support Functions</title>
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<para>
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The TSM handler function returns a palloc'd <type>TsmRoutine</> struct
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containing pointers to the support functions described below. Most of
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the functions are required, but some are optional, and those pointers can
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be NULL.
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</para>
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<para>
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<programlisting>
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void
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SampleScanGetSampleSize (PlannerInfo *root,
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RelOptInfo *baserel,
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List *paramexprs,
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BlockNumber *pages,
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double *tuples);
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</programlisting>
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This function is called during planning. It must estimate the number of
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relation pages that will be read during a sample scan, and the number of
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tuples that will be selected by the scan. (For example, these might be
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determined by estimating the sampling fraction, and then multiplying
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the <literal>baserel->pages</> and <literal>baserel->tuples</>
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numbers by that, being sure to round the results to integral values.)
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The <literal>paramexprs</> list holds the expression(s) that are
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parameters to the <literal>TABLESAMPLE</> clause. It is recommended to
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use <function>estimate_expression_value()</> to try to reduce these
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expressions to constants, if their values are needed for estimation
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purposes; but the function must provide size estimates even if they cannot
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be reduced, and it should not fail even if the values appear invalid
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(remember that they're only estimates of what the run-time values will be).
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The <literal>pages</> and <literal>tuples</> parameters are outputs.
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</para>
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<para>
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<programlisting>
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void
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InitSampleScan (SampleScanState *node,
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int eflags);
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</programlisting>
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Initialize for execution of a SampleScan plan node.
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This is called during executor startup.
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It should perform any initialization needed before processing can start.
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The <structname>SampleScanState</> node has already been created, but
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its <structfield>tsm_state</> field is NULL.
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The <function>InitSampleScan</> function can palloc whatever internal
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state data is needed by the sampling method, and store a pointer to
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it in <literal>node->tsm_state</>.
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Information about the table to scan is accessible through other fields
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of the <structname>SampleScanState</> node (but note that the
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<literal>node->ss.ss_currentScanDesc</> scan descriptor is not set
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up yet).
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<literal>eflags</> contains flag bits describing the executor's
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operating mode for this plan node.
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</para>
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<para>
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When <literal>(eflags & EXEC_FLAG_EXPLAIN_ONLY)</> is true,
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the scan will not actually be performed, so this function should only do
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the minimum required to make the node state valid for <command>EXPLAIN</>
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and <function>EndSampleScan</>.
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</para>
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<para>
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This function can be omitted (set the pointer to NULL), in which case
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<function>BeginSampleScan</> must perform all initialization needed
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by the sampling method.
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</para>
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<para>
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<programlisting>
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void
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BeginSampleScan (SampleScanState *node,
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Datum *params,
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int nparams,
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uint32 seed);
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</programlisting>
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Begin execution of a sampling scan.
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This is called just before the first attempt to fetch a tuple, and
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may be called again if the scan needs to be restarted.
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Information about the table to scan is accessible through fields
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of the <structname>SampleScanState</> node (but note that the
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<literal>node->ss.ss_currentScanDesc</> scan descriptor is not set
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up yet).
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The <literal>params</> array, of length <literal>nparams</>, contains the
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values of the parameters supplied in the <literal>TABLESAMPLE</> clause.
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These will have the number and types specified in the sampling
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method's <literal>parameterTypes</literal> list, and have been checked
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to not be null.
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<literal>seed</> contains a seed to use for any random numbers generated
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within the sampling method; it is either a hash derived from the
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<literal>REPEATABLE</> value if one was given, or the result
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of <literal>random()</> if not.
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</para>
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<para>
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This function may adjust the fields <literal>node->use_bulkread</>
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and <literal>node->use_pagemode</>.
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If <literal>node->use_bulkread</> is <literal>true</>, which it is by
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default, the scan will use a buffer access strategy that encourages
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recycling buffers after use. It might be reasonable to set this
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to <literal>false</> if the scan will visit only a small fraction of the
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table's pages.
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If <literal>node->use_pagemode</> is <literal>true</>, which it is by
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default, the scan will perform visibility checking in a single pass for
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all tuples on each visited page. It might be reasonable to set this
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to <literal>false</> if the scan will select only a small fraction of the
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tuples on each visited page. That will result in fewer tuple visibility
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checks being performed, though each one will be more expensive because it
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will require more locking.
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</para>
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<para>
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If the sampling method is
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marked <literal>repeatable_across_scans</literal>, it must be able to
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select the same set of tuples during a rescan as it did originally, that is
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a fresh call of <function>BeginSampleScan</> must lead to selecting the
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same tuples as before (if the <literal>TABLESAMPLE</> parameters
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and seed don't change).
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</para>
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<para>
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<programlisting>
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BlockNumber
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NextSampleBlock (SampleScanState *node);
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</programlisting>
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Returns the block number of the next page to be scanned, or
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<literal>InvalidBlockNumber</> if no pages remain to be scanned.
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</para>
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<para>
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This function can be omitted (set the pointer to NULL), in which case
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the core code will perform a sequential scan of the entire relation.
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Such a scan can use synchronized scanning, so that the sampling method
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cannot assume that the relation pages are visited in the same order on
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each scan.
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</para>
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<para>
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<programlisting>
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OffsetNumber
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NextSampleTuple (SampleScanState *node,
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BlockNumber blockno,
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OffsetNumber maxoffset);
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</programlisting>
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Returns the offset number of the next tuple to be sampled on the
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specified page, or <literal>InvalidOffsetNumber</> if no tuples remain to
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be sampled. <literal>maxoffset</> is the largest offset number in use
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on the page.
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</para>
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<note>
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<para>
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<function>NextSampleTuple</> is not explicitly told which of the offset
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numbers in the range <literal>1 .. maxoffset</> actually contain valid
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tuples. This is not normally a problem since the core code ignores
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requests to sample missing or invisible tuples; that should not result in
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any bias in the sample. However, if necessary, the function can
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examine <literal>node->ss.ss_currentScanDesc->rs_vistuples[]</>
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to identify which tuples are valid and visible. (This
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requires <literal>node->use_pagemode</> to be <literal>true</>.)
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</para>
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</note>
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<note>
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<para>
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<function>NextSampleTuple</> must <emphasis>not</> assume
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that <literal>blockno</> is the same page number returned by the most
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recent <function>NextSampleBlock</> call. It was returned by some
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previous <function>NextSampleBlock</> call, but the core code is allowed
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to call <function>NextSampleBlock</> in advance of actually scanning
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pages, so as to support prefetching. It is OK to assume that once
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sampling of a given page begins, successive <function>NextSampleTuple</>
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calls all refer to the same page until <literal>InvalidOffsetNumber</> is
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returned.
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</para>
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</note>
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<para>
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<programlisting>
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void
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EndSampleScan (SampleScanState *node);
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</programlisting>
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End the scan and release resources. It is normally not important
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to release palloc'd memory, but any externally-visible resources
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should be cleaned up.
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This function can be omitted (set the pointer to NULL) in the common
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case where no such resources exist.
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</para>
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</sect1>
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</chapter>
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