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Extend index AM API for parallel index scans.

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This patch doesn't actually make any index AM parallel-aware, but it
provides the necessary functions at the AM layer to do so.

Rahila Syed, Amit Kapila, Robert Haas
This commit is contained in:
Robert Haas
2017-01-24 16:42:58 -05:00
parent 587cda35ca
commit 7b4ac19982
14 changed files with 262 additions and 5 deletions
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67
doc/src/sgml/indexam.sgml
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@ -131,6 +131,11 @@ typedef struct IndexAmRoutine
amendscan_function amendscan;
ammarkpos_function ammarkpos; /* can be NULL */
amrestrpos_function amrestrpos; /* can be NULL */
/* interface functions to support parallel index scans */
amestimateparallelscan_function amestimateparallelscan; /* can be NULL */
aminitparallelscan_function aminitparallelscan; /* can be NULL */
amparallelrescan_function amparallelrescan; /* can be NULL */
} IndexAmRoutine;
</programlisting>
</para>
@ -624,6 +629,68 @@ amrestrpos (IndexScanDesc scan);
the <structfield>amrestrpos</> field in its <structname>IndexAmRoutine</>
struct may be set to NULL.
</para>
<para>
In addition to supporting ordinary index scans, some types of index
may wish to support <firstterm>parallel index scans</>, which allow
multiple backends to cooperate in performing an index scan. The
index access method should arrange things so that each cooperating
process returns a subset of the tuples that would be performed by
an ordinary, non-parallel index scan, but in such a way that the
union of those subsets is equal to the set of tuples that would be
returned by an ordinary, non-parallel index scan. Furthermore, while
there need not be any global ordering of tuples returned by a parallel
scan, the ordering of that subset of tuples returned within each
cooperating backend must match the requested ordering. The following
functions may be implemented to support parallel index scans:
</para>
<para>
<programlisting>
Size
amestimateparallelscan (void);
</programlisting>
Estimate and return the number of bytes of dynamic shared memory which
the access method will be needed to perform a parallel scan. (This number
is in addition to, not in lieu of, the amount of space needed for
AM-independent data in <structname>ParallelIndexScanDescData</>.)
</para>
<para>
It is not necessary to implement this function for access methods which
do not support parallel scans or for which the number of additional bytes
of storage required is zero.
</para>
<para>
<programlisting>
void
aminitparallelscan (void *target);
</programlisting>
This function will be called to initialize dynamic shared memory at the
beginning of a parallel scan. <parameter>target</> will point to at least
the number of bytes previously returned by
<function>amestimateparallelscan</>, and this function may use that
amount of space to store whatever data it wishes.
</para>
<para>
It is not necessary to implement this function for access methods which
do not support parallel scans or in cases where the shared memory space
required needs no initialization.
</para>
<para>
<programlisting>
void
amparallelrescan (IndexScanDesc scan);
</programlisting>
This function, if implemented, will be called when a parallel index scan
must be restarted. It should reset any shared state set up by
<function>aminitparallelscan</> such that the scan will be restarted from
the beginning.
</para>
</sect1>
<sect1 id="index-scanning">