(B)0[mindex-storage-clause =
[4mSTORE[m qw> index-within-clause qqqqqqqqqqqqqqqqqqqqqqqqqqwqk
m> [4mUSING[m qw> field-name qw> index-within-clause qj x
mqqqqq , <qqqqqj x
lqqqqqqqqqqqqqqqqqqqqqqqq<qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqj
mqwqqqqqqqqqqqqq>qqqqqqqqqqqqqqwqq>
m> default-threshold-clause qj
Allows you to specify a storage map definition for the index. You
can use the index-storage-clause only in a multifile database.
This clause allows you to specify which storage area files will
be used to store the index entries:
o All index entries can be associated with a single storage
area.
o Index entries can be systematically distributed, or
partitioned, among several storage areas by specifying upper
limits on the values for a key in a particular storage area.
If you omit the storage map definition, the default is to store
all the entries for an index in the main RDB$SYSTEM storage area.
You should define a storage area for an index that matches the
storage map for the relation with which it is associated. In
particular, under the following conditions, the database system
will store the index entry for a row on or near the same data
page that contains the actual row:
o The storage areas for a relation have a mixed page format.
o You specify an identical storage clause for the index as
exists in the storage map for the relation.
o The storage map for the relation also names the index in the
PLACEMENT VIA INDEX clause.
Such coincidental clustering of indexes and rows can reduce
input/output operations. With hashed indexes and coincidental
clustering, the database system can retrieve rows for exact match
queries in one input/output operation.
For sorted indexes, specifying an identical storage map reduces
input/output contention on index nodes.
1 – index-within-clause
(B)0[mindex-within-clause =
[4mWITHIN[m qwq> area-name qwqqqqqqqqqqqqqq>qqqqqqqqqqqqqqqwqk
x m> [4mWITH[m [4mLIMIT[m OF qw> literal qwj x
x mqqqq , <qqqj x
x lqqqqqqqqqqqqqqqqqqqqqqqq<qqqqqqqqqqqqqqqqqqqqj
x mqwqqqqqqqqqq>qqqqqqqqqqwqqwqqqqqqqqq>
x mq> threshold-clause qj x
mqqqqqqqqqqqqq ; <qqqqqqqqqqqj
1.1 – area-name
The name of the storage area in which you want the index stored.
You must have defined this storage area with either the DEFINE
DATABASE statement or the DEFINE STORAGE AREA clause of the
CHANGE DATABASE statement before you refer to it in the map-
storage-clause clause.
If the index is a hashed index, the storage area must have a
MIXED page format.
1.2 – WITH_LIMIT_OF
The maximum value for the index key that will reside in the
specified storage area. For multisegmented index keys, specify
a literal value for each field.
The number of literals in this clause must be less than or equal
to the number of fields in the USING clause. Repeat this clause
to partition the index entries among multiple storage areas.
When you are modifying a multisegmented index using
multisegmented keys and use the STORE USING...WITH LIMITS clause,
if the values for the first key are all the same, then set the
limit for the first key at that value. By doing this, you ensure
that the value of the second key determines the storage area in
which each record will be stored. Note that the last storage area
you specify CANNOT have a WITH LIMIT OF clause associated with
it.
1.3 – threshold-clause
(B)0[mthreshold-clause=
qq> [4mTHRESHOLDS[m ARE qqqqqqqqqqk
lqqqqqqqqqqqqq<qqqqqqqqqqqqqj
mq> ( qq> val1 qwqqqqqqqqqqq>qqqqqqqqqqwq> ) qq>
mq> ,val2 qqwqqqqq>qqqqu
mq> ,val3 qj
Specifies associated threshold values for each storage area with
uniform format that is specified in the index-within-clause. By
setting threshold values, you can make sure that Oracle Rdb does not
overlook a page with sufficient space to store compressed data.
The threshold values specify when the page is marked as FULL
in the SPAM page free space inventory lists. For example,
if you set default values of 70, 85, and 95 percent, ranges
of guaranteed free space on each data page are 30, 15, and 5
percent, respectively. If you do not set default values, the
values are (0,0,0). With values of (0,0,0), Oracle Rdb will use the
record length when setting the SPAM fullness. Oracle Rdb will never
store a record on a page at threshold 3. The value you set for
the highest threshold can be used to reserve space on the page
for future record growth.
If you specify a value of 40 for the "val1" parameter, but do not
specify values for the "val2" or "val3" parameters, the threshold
values will be set at (40,100,100).
If you use data compression, you should use logical area
thresholds to obtain optimum storage performance.
2 – field-name
The name of the field whose value will be used as a limit for
partitioning the index across multiple storage areas.
3 – default-threshold-clause
(B)0[mdefault-threshold-clause =
qq> [4mDEFAULT[m [4mTHRESHOLDS[m ARE qqk
lqqqqqqqqqqqqq<qqqqqqqqqqqqqj
mq> ( qq> val1 qwqqqqqqqqqqq>qqqqqqqqqqwq> ) qq>
mq> ,val2 qqwqqqqq>qqqqu
mq> ,val3 qj
Specifies associated threshold values for each storage area with
uniform format that is NOT specified in the index-within-clause.
By setting threshold values, you can make sure that Oracle Rdb does
not overlook a page with sufficient space to store compressed
data.
The threshold values specify when the page is marked as FULL
in the SPAM page free space inventory lists. For example,
if you set default values of 70, 85, and 95 percent, ranges
of guaranteed free space on each data page are 30, 15, and 5
percent, respectively. If you do not set default values, the
values are (0,0,0). With values of (0,0,0), Oracle Rdb will use the
record length when setting the SPAM fullness. Oracle Rdb will never
store a record on a page at threshold 3. The value you set for
the highest threshold can be used to reserve space on the page
for future record growth.
If you specify a value of 40 for the "val1" parameter, but do not
specify values for the "val2" or "val3" parameters, the threshold
values will be set at (40,100,100).
If you use data compression, you should use logical area
thresholds to obtain optimum storage performance.