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Document the new custom scan APIs. 

These APIs changed somewhat subsequent to the initial commit, and may
change further in the future, but let's document what we have today.

KaiGai Kohei and Robert Haas, reviewed by Tom Lane and Thom Brown
This commit is contained in:
Robert Haas 2015-03-13 07:55:39 -04:00
parent ebc0f5e01d
commit 82fe8b1119
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<!-- doc/src/sgml/custom-scan.sgml -->
<chapter id="custom-scan">
<title>Writing A Custom Scan Provider</title>
<indexterm zone="custom-scan">
<primary>custom scan provider</primary>
<secondary>handler for</secondary>
</indexterm>
<para>
<productname>PostgreSQL</> supports a set of experimental facilities which
are intended to allow extension modules to add new scan types to the system.
Unlike a <link linkend="fdwhandler">foreign data wrapper</>, which is only
responsible for knowing how to scan its own foreign tables, a custom scan
provider can provide an alternative method of scanning any relation in the
system. Typically, the motivation for writing a custom scan provider will
be to allow the use of some optimization not supported by the core
system, such as caching or some form of hardware acceleration. This chapter
outlines how to write a new custom scan provider.
</para>
<para>
Implementing a new type of custom scan is a three-step process. First,
during planning, it is necessary to generate access paths representing a
scan using the proposed strategy. Second, if one of those access paths
is selected by the planner as the optimal strategy for scanning a
particular relation, the access path must be converted to a plan.
Finally, it must be possible to execute the plan and generate the same
results that would have been generated for any other access path targeting
the same relation.
</para>
<sect1 id="custom-scan-path">
<title>Implementing Custom Paths</title>
<para>
A custom scan provider will typically add paths by setting the following
hook, which is called after the core code has generated what it believes
to be the complete and correct set of access paths for the relation.
<programlisting>
typedef void (*set_rel_pathlist_hook_type) (PlannerInfo *root,
RelOptInfo *rel,
Index rti,
RangeTblEntry *rte);
extern PGDLLIMPORT set_rel_pathlist_hook_type set_rel_pathlist_hook;
</programlisting>
</para>
<para>
Although this hook function can be used to examine, modify, or remove
paths generated by the core system, a custom scan provider will typically
confine itself to generating <structname>CustomPath</> objects and adding
them to <literal>rel</> using <function>add_path</>. The custom scan
provider is responsible for initializing the <structname>CustomPath</>
object, which is declared like this:
<programlisting>
typedef struct CustomPath
{
Path path;
uint32 flags;
List *custom_private;
const CustomPathMethods *methods;
} CustomPath;
</programlisting>
</para>
<para>
<structfield>path</> must be initialized as for any other path, including
the row-count estimate, start and total cost, and sort ordering provided
by this path. <structfield>flags</> is a bitmask, which should include
<literal>CUSTOMPATH_SUPPORT_BACKWARD_SCAN</> if the custom path can support
a backward scan and <literal>CUSTOMPATH_SUPPORT_MARK_RESTORE</> if it
can support mark and restore. Both capabilities are optional.
<structfield>custom_private</> can be used to store the custom path's
private data. Private data should be stored in a form that can be handled
by <literal>nodeToString</>, so that debugging routines which attempt to
print the custom path will work as designed. <structfield>methods</> must
point to a (usually statically allocated) object implementing the required
custom path methods, of which there are currently only two, as further
detailed below.
</para>
<sect2 id="custom-scan-path-callbacks">
<title>Custom Path Callbacks</title>
<para>
<programlisting>
Plan *(*PlanCustomPath) (PlannerInfo *root,
RelOptInfo *rel,
CustomPath *best_path,
List *tlist,
List *clauses);
</programlisting>
Convert a custom path to a finished plan. The return value will generally
be a <literal>CustomScan</> object, which the callback must allocate and
initialize. See <xref linkend="custom-scan-plan"> for more details.
</para>
<para>
<programlisting>
void (*TextOutCustomPath) (StringInfo str,
const CustomPath *node);
</programlisting>
Generate additional output when <function>nodeToString</> is invoked on
this custom path. This callback is optional. Since
<function>nodeToString</> will automatically dump all fields in the
structure that it can see, including <structfield>custom_private</>, this
is only useful if the <structname>CustomPath</> is actually embedded in a
larger struct containing additional fields.
</para>
</sect2>
</sect1>
<sect1 id="custom-scan-plan">
<title>Implementing Custom Plans</title>
<para>
A custom scan is represented in a finished plan tree using the following
structure:
<programlisting>
typedef struct CustomScan
{
Scan scan;
uint32 flags;
List *custom_exprs;
List *custom_private;
const CustomScanMethods *methods;
} CustomScan;
</programlisting>
</para>
<para>
<structfield>scan</> must be initialized as for any other scan, including
estimated costs, target lists, qualifications, and so on.
<structfield>flags</> is a bitmask with the same meaning as in
<structname>CustomPath</>. <structfield>custom_exprs</> should be used to
store expression trees that will need to be fixed up by
<filename>setrefs.c</> and <filename>subselect.c</>, while
<literal>custom_private</> should be used to store other private data that
is only used by the custom scan provider itself. Plan trees must be able
to be duplicated using <function>copyObject</>, so all the data stored
within these two fields must consist of nodes that function can handle.
<structfield>methods</> must point to a (usually statically allocated)
object implementing the required custom scan methods, which are further
detailed below.
</para>
<sect2 id="custom-scan-plan-callbacks">
<title>Custom Scan Callbacks</title>
<para>
<programlisting>
Node *(*CreateCustomScanState) (CustomScan *cscan);
</programlisting>
Allocate a <structname>CustomScanState</> for this
<structname>CustomScan</>. The actual allocation will often be larger than
required for an ordinary <structname>CustomScanState</>, because many
scan types will wish to embed that as the first field of a large structure.
The value returned must have the node tag and <structfield>methods</>
set appropriately, but the other fields need not be initialized at this
stage; after <function>ExecInitCustomScan</> performs basic initialization,
the <function>BeginCustomScan</> callback will be invoked to give the
custom scan state a chance to do whatever else is needed.
</para>
<para>
<programlisting>
void (*TextOutCustomScan) (StringInfo str,
const CustomScan *node);
</programlisting>
Generate additional output when <function>nodeToString</> is invoked on
this custom plan. This callback is optional. Since a
<structname>CustomScan</> must be copyable by <function>copyObject</>,
custom scan providers cannot substitute a larger structure that embeds a
<structname>CustomScan</> for the structure itself, as would be possible
for a <structname>CustomPath</> or <structname>CustomScanState</>.
Therefore, providing this callback is unlikely to be useful.
</para>
</sect2>
</sect1>
<sect1 id="custom-scan-scan">
<title>Implementing Custom Scans</title>
<para>
When a <structfield>CustomScan</> is executed, its execution state is
represented by a <structfield>CustomScanState</>, which is declared as
follows.
<programlisting>
typedef struct CustomScanState
{
ScanState ss;
uint32 flags;
const CustomExecMethods *methods;
} CustomScanState;
</programlisting>
</para>
<para>
<structfield>ss</> must be initialized as for any other scanstate;
<structfield>flags</> is a bitmask with the same meaning as in
<structname>CustomPath</> and <structname>CustomScan</>.
<structfield>methods</> must point to a (usually statically allocated)
object implementing the required custom scan state methods, which are
further detailed below. Typically, a <structname>CustomScanState</>, which
need not support <function>copyObject</>, will actually be a larger
structure embedding the above as its first member.
</para>
<sect2 id="custom-scan-scan-callbacks">
<title>Custom Execution-Time Callbacks</title>
<para>
<programlisting>
void (*BeginCustomScan) (CustomScanState *node,
EState *estate,
int eflags);
</programlisting>
Complete initialization of the supplied <structname>CustomScanState</>.
Some initialization is performed by <function>ExecInitCustomScan</>, but
any private fields should be initialized here.
</para>
<para>
<programlisting>
TupleTableSlot *(*ExecCustomScan) (CustomScanState *node);
</programlisting>
Fetch the next scan tuple. If any tuples remain, it should fill
<literal>ps_ResultTupleSlot</> with the next tuple in the current scan
direction, and then return the tuple slot. If not,
<literal>NULL</> or an empty slot should be returned.
</para>
<para>
<programlisting>
void (*EndCustomScan) (CustomScanState *node);
</programlisting>
Clean up any private data associated with the <literal>CustomScanState</>.
This method is required, but may not need to do anything if the associated
data does not exist or will be cleaned up automatically.
</para>
<para>
<programlisting>
void (*ReScanCustomScan) (CustomScanState *node);
</programlisting>
Rewind the current scan to the beginning and prepare to rescan the
relation.
</para>
<para>
<programlisting>
void (*MarkPosCustomScan) (CustomScanState *node);
</programlisting>
Save the current scan position so that it can subsequently be restored
by the <function>RestrPosCustomScan</> callback. This calback is optional,
and need only be supplied if
<literal>CUSTOMPATH_SUPPORT_MARK_RESTORE</> flag is set.
</para>
<para>
<programlisting>
void (*RestrPosCustomScan) (CustomScanState *node);
</programlisting>
Restore the previous scan position as saved by the
<function>MarkPosCustomScan</> callback. This callback is optional,
and need only be supplied if
<literal>CUSTOMPATH_SUPPORT_MARK_RESTORE</> flag is set.
</para>
<para>
<programlisting>
void (*ExplainCustomScan) (CustomScanState *node,
List *ancestors,
ExplainState *es);
</programlisting>
Output additional information on <command>EXPLAIN</> that involves
custom-scan node. This callback is optional. Common data stored in the
<structname>ScanState</>, such as the target list and scan relation, will
be shown even without this callback, but the callback allows the display
of additional, private state.
</para>
</sect2>
</sect1>
</chapter>

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<!ENTITY nls SYSTEM "nls.sgml">
<!ENTITY plhandler SYSTEM "plhandler.sgml">
<!ENTITY fdwhandler SYSTEM "fdwhandler.sgml">
<!ENTITY custom-scan SYSTEM "custom-scan.sgml">
<!ENTITY logicaldecoding SYSTEM "logicaldecoding.sgml">
<!ENTITY protocol SYSTEM "protocol.sgml">
<!ENTITY sources SYSTEM "sources.sgml">

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doc/src/sgml/postgres.sgml
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&nls;
&plhandler;
&fdwhandler;
&custom-scan;
&geqo;
&indexam;
&gist;