@@ -1680,6 +1680,15 @@ typedef struct
16801680 itself.
16811681 </para>
16821682
1683+ <para>
1684+ Another important point is to avoid leaving any uninitialized bits
1685+ within data type values; for example, take care to zero out any
1686+ alignment padding bytes that might be present in structs. Without
1687+ this, logically-equivalent constants of your data type might be
1688+ seen as unequal by the planner, leading to inefficient (though not
1689+ incorrect) plans.
1690+ </para>
1691+
16831692 <warning>
16841693 <para>
16851694 <emphasis>Never</> modify the contents of a pass-by-reference input
@@ -1723,7 +1732,7 @@ typedef struct {
17231732char buffer[40]; /* our source data */
17241733...
17251734text *destination = (text *) palloc(VARHDRSZ + 40);
1726- destination->length = VARHDRSZ + 40;
1735+ SET_VARSIZE( destination, VARHDRSZ + 40) ;
17271736memcpy(destination->data, buffer, 40);
17281737...
17291738]]>
@@ -1732,6 +1741,8 @@ memcpy(destination->data, buffer, 40);
17321741 <literal>VARHDRSZ</> is the same as <literal>sizeof(int4)</>, but
17331742 it's considered good style to use the macro <literal>VARHDRSZ</>
17341743 to refer to the size of the overhead for a variable-length type.
1744+ Also, the length field <emphasis>must</> be set using the
1745+ <literal>SET_VARSIZE</> macro, not by simple assignment.
17351746 </para>
17361747
17371748 <para>
@@ -2345,13 +2356,16 @@ concat_text(PG_FUNCTION_ARGS)
23452356
23462357 <listitem>
23472358 <para>
2348- Always zero the bytes of your structures using
2349- <function>memset</function>. Without this, it's difficult to
2359+ Always zero the bytes of your structures using <function>memset</>
2360+ (or allocate them with <function>palloc0</> in the first place).
2361+ Even if you assign to each field of your structure, there might be
2362+ alignment padding (holes in the structure) that contain
2363+ garbage values. Without this, it's difficult to
23502364 support hash indexes or hash joins, as you must pick out only
23512365 the significant bits of your data structure to compute a hash.
2352- Even if you initialize all fields of your structure, there might be
2353- alignment padding (holes in the structure) that contain
2354- garbage values.
2366+ The planner also sometimes relies on comparing constants via
2367+ bitwise equality, so you can get undesirable planning results if
2368+ logically-equivalent values aren't bitwise equal .
23552369 </para>
23562370 </listitem>
23572371