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Overview
Comment:More fixes to the virtual table documentation.
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SHA3-256: 70c4eaad4d2748d788dcd8f5870af441cdf4b80c6518393f768e2c6286ba60cf
User & Date: drh 2018-11-16 14:33:01.826
Context
2018-11-16
14:35
Add the SQLITE_CONSTRAINT return from xBestIndex to the change log. (check-in: 14b1451274 user: drh tags: trunk)
14:33
More fixes to the virtual table documentation. (check-in: 70c4eaad4d user: drh tags: trunk)
14:30
Explain the use of SQLITE_CONSTRAINT as a return value from xBestIndex. (check-in: 52c277f049 user: drh tags: trunk)
Changes
Unified Diff Ignore Whitespace Patch
Changes to pages/vtab.in.
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<li><p>Scan all rows of tablevalued func and for each row find rows
in realtab where x is equal to tablevaluedfunc.param1.
</ol>

<p>The xBestIndex method will be invoked once for each of the potential
plans above.  For plan 1, the aConstraint[].usable flag for for the
SQLITE_CONSTRAINT_EQ constraint on the param1 column will be true because
the right-hand side value for the "param1 = ?" constraint will be known.

But for plan 2, the aConstraint[].usable flag for "param1 = ?" will be false
because the right-hand side value is determined by an inner loop.  Hence,

the xBestIndex method should return SQLITE_CONSTRAINT when presented with
plan 2, forcing SQLite to choose plan 1.


<tcl>hd_fragment xdisconnect {sqlite3_module.xDisconnect} {xDisconnect}</tcl>
<h2>The xDisconnect Method</h2>

<codeblock>
  int (*xDisconnect)(sqlite3_vtab *pVTab);
</codeblock>








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<li><p>Scan all rows of tablevalued func and for each row find rows
in realtab where x is equal to tablevaluedfunc.param1.
</ol>

<p>The xBestIndex method will be invoked once for each of the potential
plans above.  For plan 1, the aConstraint[].usable flag for for the
SQLITE_CONSTRAINT_EQ constraint on the param1 column will be true because
the right-hand side value for the "param1 = ?" constraint will be known,
since it is determined by the outer realtab loop.
But for plan 2, the aConstraint[].usable flag for "param1 = ?" will be false
because the right-hand side value is determined by an inner loop and is thus
an unknown quantity.  Because param1 is a required input to the table-valued
functions, the xBestIndex method should return SQLITE_CONSTRAINT when presented 
with plan 2, indicating that a required input is missing.  This forces the
query planner to select plan 1.

<tcl>hd_fragment xdisconnect {sqlite3_module.xDisconnect} {xDisconnect}</tcl>
<h2>The xDisconnect Method</h2>

<codeblock>
  int (*xDisconnect)(sqlite3_vtab *pVTab);
</codeblock>
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