Many hyperlinks are disabled.
Use anonymous login
to enable hyperlinks.
Overview
| Comment: | documentation updates (CVS 112) |
|---|---|
| Downloads: | Tarball | ZIP archive | SQL archive |
| Timelines: | family | ancestors | descendants | both | trunk |
| Files: | files | file ages | folders |
| SHA1: |
c686c6076abadcb715fe74436fa8bab4 |
| User & Date: | drh 2000-07-30 20:04:43 |
Context
|
2000-07-31
| ||
| 11:57 | fix a configuration problem on TAC-4 (CVS 113) (check-in: 69b73ea3 user: drh tags: trunk) | |
|
2000-07-30
| ||
| 20:04 | documentation updates (CVS 112) (check-in: c686c607 user: drh tags: trunk) | |
|
2000-07-29
| ||
| 13:20 | better column names in the shell (CVS 111) (check-in: 57022a9d user: drh tags: trunk) | |
Changes
Changes to src/vdbe.c.
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
....
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
|
** inplicit conversion from one type to the other occurs as necessary. ** ** Most of the code in this file is taken up by the sqliteVdbeExec() ** function which does the work of interpreting a VDBE program. ** But other routines are also provided to help in building up ** a program instruction by instruction. ** ** $Id: vdbe.c,v 1.36 2000/07/29 13:06:59 drh Exp $ */ #include "sqliteInt.h" #include <unistd.h> /* ** SQL is translated into a sequence of instructions to be ** executed by a virtual machine. Each instruction is an instance ................................................................................ PopStack(p, 1); break; } /* Opcode: KeyAsData P1 P2 * ** ** Turn the key-as-data mode for cursor P1 either on (if P2==1) or ** off (if P2==0). In key-as-data mode, the OP_Fetch opcode pulls ** data off of the key rather than the data. This is useful for ** processing compound selects. */ case OP_KeyAsData: { int i = pOp->p1; if( i>=0 && i<p->nCursor && p->aCsr[i].pCursor!=0 ){ p->aCsr[i].keyAsData = pOp->p2; |
|
|
|
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
....
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
|
** inplicit conversion from one type to the other occurs as necessary. ** ** Most of the code in this file is taken up by the sqliteVdbeExec() ** function which does the work of interpreting a VDBE program. ** But other routines are also provided to help in building up ** a program instruction by instruction. ** ** $Id: vdbe.c,v 1.37 2000/07/30 20:04:43 drh Exp $ */ #include "sqliteInt.h" #include <unistd.h> /* ** SQL is translated into a sequence of instructions to be ** executed by a virtual machine. Each instruction is an instance ................................................................................ PopStack(p, 1); break; } /* Opcode: KeyAsData P1 P2 * ** ** Turn the key-as-data mode for cursor P1 either on (if P2==1) or ** off (if P2==0). In key-as-data mode, the OP_Field opcode pulls ** data off of the key rather than the data. This is useful for ** processing compound selects. */ case OP_KeyAsData: { int i = pOp->p1; if( i>=0 && i<p->nCursor && p->aCsr[i].pCursor!=0 ){ p->aCsr[i].keyAsData = pOp->p2; |
Changes to www/opcode.tcl.
1
2
3
4
5
6
7
8
9
10
11
..
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
|
#
# Run this Tcl script to generate the sqlite.html file.
#
set rcsid {$Id: opcode.tcl,v 1.3 2000/06/23 17:02:09 drh Exp $}
puts {<html>
<head>
<title>SQLite Virtual Machine Opcodes</title>
</head>
<body bgcolor=white>
<h1 align=center>
................................................................................
<p>In order to execute an SQL statement, the SQLite library first parses
the SQL, analyzes the statement, then generates a short program to execute
the statement. The program is generated for a "virtual machine" implemented
by the SQLite library. This document describes the operation of that
virtual machine.</p>
<p>This document is intended as a reference, not a tutorial.
A separate <a href="vdbe.html">Virtual Machine Tutorial</a> is currently
in preparation. If you are looking for a narrative description
of how the virtual machine works, you should read the tutorial
and not this document. Once you have a basic idea of what the
virtual machine does, you can refer back to this document for
the details on a particular opcode.</p>
<p>The source code to the virtual machine is in the <b>vdbe.c</b> source
file. All of the opcode definitions further down in this document are
|
|
|
|
|
1
2
3
4
5
6
7
8
9
10
11
..
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
|
#
# Run this Tcl script to generate the sqlite.html file.
#
set rcsid {$Id: opcode.tcl,v 1.4 2000/07/30 20:04:43 drh Exp $}
puts {<html>
<head>
<title>SQLite Virtual Machine Opcodes</title>
</head>
<body bgcolor=white>
<h1 align=center>
................................................................................
<p>In order to execute an SQL statement, the SQLite library first parses
the SQL, analyzes the statement, then generates a short program to execute
the statement. The program is generated for a "virtual machine" implemented
by the SQLite library. This document describes the operation of that
virtual machine.</p>
<p>This document is intended as a reference, not a tutorial.
A separate <a href="vdbe.html">Virtual Machine Tutorial</a> is
available. If you are looking for a narrative description
of how the virtual machine works, you should read the tutorial
and not this document. Once you have a basic idea of what the
virtual machine does, you can refer back to this document for
the details on a particular opcode.</p>
<p>The source code to the virtual machine is in the <b>vdbe.c</b> source
file. All of the opcode definitions further down in this document are
|
Changes to www/vdbe.tcl.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 .. 92 93 94 95 96 97 98 99 100 101 102 103 104 105 ... 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 ... 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 ... 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 ... 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 |
#
# Run this Tcl script to generate the vdbe.html file.
#
set rcsid {$Id: vdbe.tcl,v 1.4 2000/07/28 14:32:51 drh Exp $}
puts {<html>
<head>
<title>The Virtual Database Engine of SQLite</title>
</head>
<body bgcolor=white>
<h1 align=center>
The Virtual Database Engine of SQLite
</h1>}
puts "<p align=center>
(This page was last modified on [lrange $rcsid 3 4] GMT)
</p>"
puts {
<blockquote><font color="red"><b>This document is
currently under development. It is incomplete and contains
errors. Use it accordingly.</b></font></blockquote>
}
puts {
<p>If you want to know how the SQLite library works internally,
you need to begin with a solid understanding of the Virtual Database
Engine or VDBE. The VDBE occurs right in the middle of the
processing stream (see the <a href="arch.html">architecture diagram</a>)
and so it seems to touch most parts of the library. Even
................................................................................
<blockquote><pre>
INSERT INTO examp VALUES('Hello, World!',99);
</pre></blockquote>
<p>We can see the VDBE program that SQLite uses to implement this
INSERT using the <b>sqlite</b> command-line utility. First start
up <b>sqlite</b> on a new, empty database, then create the table.
Finally, enter the INSERT statement shown above, but precede the
INSERT with the special keyword "EXPLAIN". The EXPLAIN keyword
will cause <b>sqlite</b> to print the VDBE program rather than
execute it. We have:</p>
}
proc Code {body} {
................................................................................
opened cursors must be opened with the same P2 value. It is
not allowed to have one cursor open for reading a file and
another cursor open for writing that same file.</p>
<p>The second instruction, New, generates an integer key that
has not been previously used in the file "examp". The New instruction
uses its P1 operand as the handle of a cursor for the file
for which the new key will be generated. The new key is
pushed onto the stack. The P2 and P3 operands are not used
by the New instruction.</p>
<p>The third instruction, String, simply pushes its P3
operand onto the stack. After the string instruction executes,
the stack will contain two elements, as follows:</p>
}
................................................................................
the stack looks like this:</p>
}
stack {A data record holding "Hello, World!" and 99} \
{A random integer key}
puts {<p>The last instruction pops the top two elements from the stack
and uses them as data and key to make a new entry in database
database file pointed to by cursor P1. This instruction is where
the insert actually occurs.</p>
<p>After the last instruction executes, the program counter
advances to one past the last instruction, which causes the
VDBE to halt. When the VDBE halts, it automatically closes
all open cursors, frees any elements left on the stack,
................................................................................
prototype:</p>
<blockquote><pre>
int Callback(void *pUserData, int nColumn, char *azData[], char *azColumnName[]);
</pre></blockquote>
<p>The SQLite library supplies the VDBE with a pointer to the callback function
itself, and the <b>pUserData</b> pointer. The job of the VDBE is to
come up with values for <b>nColumn</b>, <b>azData[]</b>,
and <b>azColumnName[]</b>.
<b>nColumn</b> is the number of columns in the results, of course.
<b>azColumnName[]</b> is an array of strings where each string is the name
of one of the result column. <b>azData[]</b> is an array of strings holding
the actual data.</p>
................................................................................
columns from all entries of the sqlite_master table.
The "sql" column contains the complete SQL text of the
CREATE statement that originally generated the index or
table. This text is fed back into the SQLite parser
and used to reconstruct the
internal data structures describing the index or table.</p>
<h2>Using Indexes To Speed Searches</h2>
<i>TBD</i>
<h2>Joins</h2>
<i>TBD</i>
<h2>The ORDER BY clause</h2>
<i>TBD</i>
<h2>Aggregate Functions And The GROUP BY and HAVING Clauses</h2>
<i>TBD</i>
<h2>Using SELECT Statements As Terms In An Expression</h2>
<i>TBD</i>
<h2>Compound SELECT Statements</h2>
<i>TBD</i>
}
puts {
<p><hr /></p>
<p><a href="index.html"><img src="/goback.jpg" border=0 />
Back to the SQLite Home Page</a>
</p>
</body></html>}
|
| > | | | | < > > > > | | > > | | < > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > < > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > < > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > < > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > < > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > < | > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > |
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 .. 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 ... 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 ... 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 ... 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 ... 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 1520 1521 1522 1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553 1554 1555 1556 1557 1558 1559 1560 1561 1562 1563 1564 1565 1566 1567 1568 1569 1570 1571 1572 1573 1574 1575 1576 1577 1578 1579 1580 1581 1582 1583 1584 1585 1586 1587 1588 1589 1590 1591 1592 1593 1594 1595 1596 1597 1598 1599 1600 1601 1602 1603 1604 1605 1606 1607 1608 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620 1621 1622 1623 1624 1625 |
#
# Run this Tcl script to generate the vdbe.html file.
#
set rcsid {$Id: vdbe.tcl,v 1.5 2000/07/30 20:04:43 drh Exp $}
puts {<html>
<head>
<title>The Virtual Database Engine of SQLite</title>
</head>
<body bgcolor=white>
<h1 align=center>
The Virtual Database Engine of SQLite
</h1>}
puts "<p align=center>
(This page was last modified on [lrange $rcsid 3 4] GMT)
</p>"
# puts {
# <blockquote><font color="red"><b>This document is
# currently under development. It is incomplete and contains
# errors. Use it accordingly.</b></font></blockquote>
# }
puts {
<p>If you want to know how the SQLite library works internally,
you need to begin with a solid understanding of the Virtual Database
Engine or VDBE. The VDBE occurs right in the middle of the
processing stream (see the <a href="arch.html">architecture diagram</a>)
and so it seems to touch most parts of the library. Even
................................................................................
<blockquote><pre>
INSERT INTO examp VALUES('Hello, World!',99);
</pre></blockquote>
<p>We can see the VDBE program that SQLite uses to implement this
INSERT using the <b>sqlite</b> command-line utility. First start
up <b>sqlite</b> on a new, empty database, then create the table.
Next change the output format of <b>sqlite</b> to a form that
is designed to work with VDBE program dumps by entering the
".explain" command.
Finally, enter the INSERT statement shown above, but precede the
INSERT with the special keyword "EXPLAIN". The EXPLAIN keyword
will cause <b>sqlite</b> to print the VDBE program rather than
execute it. We have:</p>
}
proc Code {body} {
................................................................................
opened cursors must be opened with the same P2 value. It is
not allowed to have one cursor open for reading a file and
another cursor open for writing that same file.</p>
<p>The second instruction, New, generates an integer key that
has not been previously used in the file "examp". The New instruction
uses its P1 operand as the handle of a cursor for the file
for which the new key will be generated. The generated key is
pushed onto the stack. The P2 and P3 operands are not used
by the New instruction.</p>
<p>The third instruction, String, simply pushes its P3
operand onto the stack. After the string instruction executes,
the stack will contain two elements, as follows:</p>
}
................................................................................
the stack looks like this:</p>
}
stack {A data record holding "Hello, World!" and 99} \
{A random integer key}
puts {<p>The last instruction pops the top two elements from the stack
and uses them as data and key to make a new entry in the
database file pointed to by cursor P1. This instruction is where
the insert actually occurs.</p>
<p>After the last instruction executes, the program counter
advances to one past the last instruction, which causes the
VDBE to halt. When the VDBE halts, it automatically closes
all open cursors, frees any elements left on the stack,
................................................................................
prototype:</p>
<blockquote><pre>
int Callback(void *pUserData, int nColumn, char *azData[], char *azColumnName[]);
</pre></blockquote>
<p>The SQLite library supplies the VDBE with a pointer to the callback function
and the <b>pUserData</b> pointer. (Both the callback and the user data were
originally passed in as argument to the <b>sqlite_exec()</b> API function.)
The job of the VDBE is to
come up with values for <b>nColumn</b>, <b>azData[]</b>,
and <b>azColumnName[]</b>.
<b>nColumn</b> is the number of columns in the results, of course.
<b>azColumnName[]</b> is an array of strings where each string is the name
of one of the result column. <b>azData[]</b> is an array of strings holding
the actual data.</p>
................................................................................
columns from all entries of the sqlite_master table.
The "sql" column contains the complete SQL text of the
CREATE statement that originally generated the index or
table. This text is fed back into the SQLite parser
and used to reconstruct the
internal data structures describing the index or table.</p>
<h2>Using Indexes To Speed Searching</h2>
<p>In the example queries above, every row of the table being
queried must be loaded off of the disk and examined, even if only
a small percentage of the rows end up in the result. This can
take a long time on a big table. To speed things up, SQLite
can use an index.</p>
<p>An GDBM file associates a key with some data. For a SQLite
table, the GDBM file is set up so that the key is a integer
and the data is the information for one row of the table.
Indices in SQLite reverse this arrangement. The GDBM key
is (some of) the information being stored and the GDBM data
is an integer.
To access a table row that has some particular
content, we first look up the content in the GDBM index file to find
its integer index, then we use that integer to look up the
complete record in the GDBM table file.</p>
<p>Note that because GDBM uses hashing instead of b-trees, indices
are only helpful when the WHERE clause of the SELECT statement
contains tests for equality. Inequalities will not work since there
is no way to ask GDBM to fetch records that do not match a key.
So, in other words, queries like the following will use an index
if it is available:</p>
<blockquote><pre>
SELECT * FROM examp WHERE two==50;
</pre></blockquote>
<p>If there exists an index that maps the "two" column of the "examp"
table into integers, then SQLite will use that index to find the integer
keys of all rows in examp that have a value of 50 for column two.
But the following query will not use an index:</p>
<blockquote><pre>
SELECT * FROM examp WHERE two<50;
</pre></blockquote>
<p>GDBM does not have the ability to select records based on
a magnitude comparison, and so there is no way to use an index
to speed the search in this case.</p>
<p>To understand better how indices work, lets first look at how
they are created. Let's go ahead and put an index on the two
column of the examp table. We have:</p>
<blockquote><pre>
CREATE INDEX examp_idx1 ON examp(two);
</pre></blockquote>
<p>The VDBE code generated by the above statement looks like the
following:</p>
}
Code {
addr opcode p1 p2 p3
---- ------------ ----- ----- ----------------------------------------
0 Open 0 0 examp
1 Open 1 1 examp_idx1
2 Open 2 1 sqlite_master
3 New 2 0
4 String 0 0 index
5 String 0 0 examp_idx1
6 String 0 0 examp
7 String 0 0 CREATE INDEX examp_idx1 ON examp(two)
8 MakeRecord 4 0
9 Put 2 0
10 Close 2 0
11 Next 0 17
12 Key 0 0
13 Field 0 1
14 MakeKey 1 0
15 PutIdx 1 0
16 Goto 0 11
17 Noop 0 0
18 Close 1 0
19 Close 0 0
}
puts {
<p>Remember that every table (except sqlite_master) and every named
index has an entry in the sqlite_master table. Since we are creating
a new index, we have to add a new entry to sqlite_master. This is
handled by instructions 2 through 10. Adding an entry to sqlite_master
works just like any other INSERT statement so we will not say anymore
about it here. In this example, we want to focus on populating the
new index with valid data, which happens on instructions 0 and 1 and
on instructions 11 through 19.</p>
<p>The first thing that happens is that we open the table being
indexed for reading. In order to construct an index for a table,
we have to know what is in that table. The second instruction
opens the index file for writing.</p>
<p>Instructions 11 through 16 implement a loop over every row
of the table being indexed. For each table row, we first extract
the integer key for that row in instruction 12, then get the
value of the two column in instruction 13. The MakeKey instruction
at 14 converts data from the two column (which is on the top of
the stack) into a valid index key. For an index on a single column,
this is basically a no-op. But if the P1 operand to MakeKey had
been greater than one multiple entries would have been popped from
the stack and converted into a single index key. The PutIdx
instruction at 15 is what actually creates the index entry. PutIdx
pops two elements from the stack. The top of the stack is used as
a key to fetch an entry from the GDBM index file. Then the integer
which was second on stack is added to the set of integers for that
index and the new record is written back to the GDBM file. Note
that the same index entry can store multiple integers if there
are two or more table entries with the same value for the two
column.
</p>
<p>Now let's look at how this index will be used. Consider the
following query:</p>
<blockquote><pre>
SELECT * FROM examp WHERE two==50;
</pre></blockquote>
<p>SQLite generates the following VDBE code to handle this query:</p>
}
Code {
addr opcode p1 p2 p3
---- ------------ ----- ----- ----------------------------------------
0 ColumnCount 2 0
1 ColumnName 0 0 one
2 ColumnName 1 0 two
3 Open 0 0 examp
4 Open 1 0 examp_idx1
5 Integer 50 0
6 MakeKey 1 0
7 Fetch 1 0
8 NextIdx 1 14
9 Fetch 0 0
10 Field 0 0
11 Field 0 1
12 Callback 2 0
13 Goto 0 8
14 Close 0 0
15 Close 1 0
}
puts {
<p>The SELECT begins in a familiar fashion. First the column
names are initialized and the table being queried is opened.
Things become different beginning with instruction 4 where
the index file is also opened. Instructions 5 and 6 make
a key with the value of 50 and instruction 7 fetches the
record of the GDBM index file that has this key. This will
be the only fetch from the index file.</p>
<p>Instructions 8 through 13 implement a loop over all
integers in the payload of the index record that was fetched
by instruction 7. The NextIdx operation works much like
the Next and ListRead operations that are discussed above.
Each NextIdx instruction reads a single integer from the
payload of the index record and falls through, except that
if there are no more records it jumps immediately to 14.</p>
<p>The Fetch instruction at 9 loads a single record from
the GDBM file that holds the table. Then there are two
Field instructions to construct the result and the callback
is invoked. All this is the same as we have seen before.
The only difference is that the loop is now constructed using
NextIdx instead of Next.</p>
<p>Since the index is used to look up values in the table,
it is important that the index and table be kept consistent.
Now that there is an index on the examp table, we will have
to update that index whenever data is inserted, deleted, or
changed in the examp table. Remember the first example above
how we were able to insert a new row into the examp table using
only 6 VDBE instructions. Now that this table is indexed, 10
instructions are required. The SQL statement is this:</p>
<blockquote><pre>
INSERT INTO examp VALUES('Hello, World!',99);
</pre></blockquote>
<p>And the generated code looks like this:</p>
}
Code {
addr opcode p1 p2 p3
---- ------------ ----- ----- ----------------------------------------
0 Open 0 1 examp
1 Open 1 1 examp_idx1
2 New 0 0
3 Dup 0 0
4 String 0 0 Hello, World!
5 Integer 99 0
6 MakeRecord 2 0
7 Put 0 0
8 Integer 99 0
9 MakeKey 1 0
10 PutIdx 1 0
}
puts {
<p>At this point, you should understand the VDBE well enough to
figure out on your own how the above program works. So we will
not discuss it further in this text.</p>
<h2>Joins</h2>
<p>In a join, two or more tables are combined to generate a single
result. The result table consists of every possible combination
of rows from the tables being joined. The easiest and most natural
way to implement this is with nested loops.</p>
<p>Recall the query template discussed above where there was a
single loop that searched through every record of the table.
In a join we have basically the same thing except that there
are nested loops. For example, to join two tables, the query
template might look something like this:</p>
<p>
<ol>
<li>Initialize the <b>azColumnName[]</b> array for the callback.</li>
<li>Open two cursors, one to each of the two tables being queried.</li>
<li>For each record in the first table, do:
<ol type="a">
<li>For each record in the second table do:
<ol type="i">
<li>If the WHERE clause evaluates to FALSE, then skip the steps that
follow and continue to the next record.</li>
<li>Compute all columns for the current row of the result.</li>
<li>Invoke the callback function for the current row of the result.</li>
</ol></li>
</ol>
<li>Close both cursors.</li>
</ol>
</p>
<p>This template will work, but it is likely to be slow since we
are now dealing with an O(N<sup>2</sup>) loop. But it often works
out that the WHERE clause can be factored into terms and that one or
more of those terms will involve only columns in the first table.
When this happens, we can factor part of the WHERE clause test out of
the inner loop and gain a lot of efficiency. So a better template
would be something like this:</p>
<p>
<ol>
<li>Initialize the <b>azColumnName[]</b> array for the callback.</li>
<li>Open two cursors, one to each of the two tables being queried.</li>
<li>For each record in the first table, do:
<ol type="a">
<li>Evaluate terms of the WHERE clause that only involve columns from
the first table. If any term is false (meaning that the whole
WHERE clause must be false) then skip the rest of this loop and
continue to the next record.</li>
<li>For each record in the second table do:
<ol type="i">
<li>If the WHERE clause evaluates to FALSE, then skip the steps that
follow and continue to the next record.</li>
<li>Compute all columns for the current row of the result.</li>
<li>Invoke the callback function for the current row of the result.</li>
</ol></li>
</ol>
<li>Close both cursors.</li>
</ol>
</p>
<p>Additional speed-up can occur if an index can be used to speed
the search of either or the two loops.</p>
<p>SQLite always constructs the loops in the same order as the
tables appear in the FROM clause of the SELECT statement. The
left-most table becomes the outer loop and the right-most table
becomes the inner loop. It is possible, in theory, to reorder
the loops in some circumstances to speed the evaluation of the
join. But SQLite does not attempt this optimization.</p>
<p>You can see how SQLite constructs nested loops in the following
example:</p>
<blockquote><pre>
CREATE TABLE examp2(three int, four int);
SELECT * FROM examp, examp2 WHERE two<50 AND four==two;
</pre></blockquote>
}
Code {
addr opcode p1 p2 p3
---- ------------ ----- ----- ----------------------------------------
0 ColumnCount 4 0
1 ColumnName 0 0 examp.one
2 ColumnName 1 0 examp.two
3 ColumnName 2 0 examp2.three
4 ColumnName 3 0 examp2.four
5 Open 0 0 examp
6 Open 1 0 examp2
7 Next 0 21
8 Field 0 1
9 Integer 50 0
10 Ge 0 7
11 Next 1 7
12 Field 1 1
13 Field 0 1
14 Ne 0 11
15 Field 0 0
16 Field 0 1
17 Field 1 0
18 Field 1 1
19 Callback 4 0
20 Goto 0 11
21 Close 0 0
22 Close 1 0
}
puts {
<p>The outer loop over table examp is implement by instructions
7 through 20. The inner loop is instructions 11 through 20.
Notice that the "two<50" term of the WHERE expression involves
only columns from the first table and can be factored out of
the inner loop. SQLite does this and implements the "two<50"
test in instructions 8 through 10. The "four==two" test is
implement by instructions 12 through 14 in the inner loop.</p>
<p>SQLite does not impose any arbitrary limits on the tables in
a join. It also allows a table to be joined with itself.</p>
<h2>The ORDER BY clause</h2>
<p>As noted previously, GDBM does not have any facility for
handling inequalities. A consequence of this is that we cannot
sort on disk using GDBM. All sorted must be done in memory.</p>
<p>SQLite implements the ORDER BY clause using a special
set of instruction control an object called a sorter. In the
inner-most loop of the query, where there would normally be
a Callback instruction, instead a record is constructed that
contains both callback parameters and a key. This record
is added to a linked list. After the query loop finishes,
the list of records is sort and this walked. For each record
on the list, the callback is invoked. Finally, the sorter
is closed and memory is deallocated.</p>
<p>We can see the process in action in the following query:</p>
<blockquote><pre>
SELECT * FROM examp ORDER BY one DESC, two;
</pre></blockquote>
}
Code {
addr opcode p1 p2 p3
---- ------------ ----- ----- ----------------------------------------
0 SortOpen 0 0
1 ColumnCount 2 0
2 ColumnName 0 0 one
3 ColumnName 1 0 two
4 Open 0 0 examp
5 Next 0 14
6 Field 0 0
7 Field 0 1
8 SortMakeRec 2 0
9 Field 0 0
10 Field 0 1
11 SortMakeKey 2 0 -+
12 SortPut 0 0
13 Goto 0 5
14 Close 0 0
15 Sort 0 0
16 SortNext 0 19
17 SortCallback 2 0
18 Goto 0 16
19 SortClose 0 0
}
puts {
<p>The sorter is opened on the first instruction. The VDBE allows
any number of sorters, but in practice no more than one is every used.</p>
<p>The query loop is built from instructions 5 through 13. Instructions
6 through 8 build a record that contains the azData[] values for a single
invocation of the callback. A sort key is generated by instructions
9 through 11. Instruction 12 combines the invocation record and the
sort key into a single entry and puts that entry on the sort list.<p>
<p>The P3 argument of instruction 11 is of particular interest. The
sort key is formed by prepending one character from P3 to each string
and concatenating all the strings. The sort comparison function will
look at this character to determine whether the sort order is
ascending or descending. In this example, the first column should be
sorted in descending order so its prefix is "-" and the second column
should sort in ascending order so its prefix is "+".</p>
<p>After the query loop ends, the table being queried is closed at
instruction 14. This is done early in order to allow other processes
or threads to access that table, if desired. The list of records
that was built up inside the query loop is sorted by the instruction
at 15. Instructions 16 through 18 walk through the record list
(which is now in sorted order) and invoke the callback once for
each record. Finally, the sorter is closed at instruction 19.</p>
<h2>Aggregate Functions And The GROUP BY and HAVING Clauses</h2>
<p>To compute aggregate functions, the VDBE implements a special
data structure and instructions for controlling that data structure.
The data structure is an unordered set of buckets, where each bucket
has a key and one or more memory locations. Within the query
loop, the GROUP BY clause is used to construct a key and the bucket
with that key is brought into focus. A new bucket is created with
the key if one did not previously exist. Once the bucket is in
focus, the memory locations of the bucket are used to accumulate
the values of the various aggregate functions. After the query
loop terminates, the each bucket is visited once to generate a
single row of the results.</p>
<p>An example will help to clarify this concept. Consider the
following query:</p>
<blockquote><pre>
SELECT three, min(three+four)+avg(four)
FROM examp2
GROUP BY three;
</pre></blockquote>
}
puts {
<p>The VDBE code generated for this query is as follows:</p>
}
Code {
addr opcode p1 p2 p3
---- ------------ ----- ----- ----------------------------------------
0 ColumnCount 2 0
1 ColumnName 0 0 three
2 ColumnName 1 0 min(three+four)+avg(four)
3 AggReset 0 4
4 Open 0 0 examp2
5 Next 0 23
6 Field 0 0
7 MakeKey 1 0
8 AggFocus 0 11
9 Field 0 0
10 AggSet 0 0
11 Field 0 0
12 Field 0 1
13 Add 0 0
14 AggGet 0 1
15 Min 0 0
16 AggSet 0 1
17 AggIncr 1 2
18 Field 0 1
19 AggGet 0 3
20 Add 0 0
21 AggSet 0 3
22 Goto 0 5
23 Close 0 0
24 AggNext 0 33
25 AggGet 0 0
26 AggGet 0 1
27 AggGet 0 3
28 AggGet 0 2
29 Divide 0 0
30 Add 0 0
31 Callback 2 0
32 Goto 0 24
33 Noop 0 0
}
puts {
<p>The first instruction of interest is the AggReset at 3.
The AggReset instruction initializes the set of buckets to be the
empty set and specifies the number of memory slots available in each
bucket. In this example, each bucket will hold four memory slots.
It is not obvious, but if you look closely at the rest of the program
you can figure out what each of these four slots is intended for.</p>
<blockquote><table border="2" cellpadding="5">
<tr><th>Memory Slot</th><th>Intended Use Of This Memory Slot</th></tr>
<tr><td>0</td><td>The "three" column -- the key to the bucket</td></tr>
<tr><td>1</td><td>The minimum "three+four" value</td></tr>
<tr><td>2</td><td>The number of records with the same key. This value
divides the value in slot 3 to compute "avg(four)".</td></tr>
<tr><td>3</td><td>The sum of all "four" values. This is used to compute
"avg(four)".</td></tr>
</table></blockquote>
<p>The query loop is implement by instructions 5 through 22.
The aggregate key specified by the GROUP BY clause is computed
by instructions 6 and 7. Instruction 8 causes the appropriate
bucket to come into focus. If a bucket with the given key does
not already exists, a new bucket is created and control falls
through to instructions 9 and 10 which initialize the bucket.
If the bucket does already exist, then a jump is made to instruction
11. The values of aggregate functions are updated by the instructions
between 11 and 21. Instructions 11 through 16 update memory
slot 1 to hold the next value "min(three+four)". The counter in
slot 2 is incremented by instruction 17. Finally the sum of
the "four" column is updated by instructions 18 through 21.</p>
<p>After the query loop is finished, the GDBM table is closed at
instruction 23 so that its lock will be released and it can be
used by other threads or processes. The next step is to loop
over all aggregate buckets and output one row of the result for
each bucket. This is done by the loop at instructions 24
through 32. The AggNext instruction at 24 brings the next bucket
into focus, or jumps to the end of the loop if all buckets have
been examined already. The first column of the result ("three")
is computed by instruction 25. The second result column
("min(three+four)+avg(four)") is computed by instructions
26 through 30. Notice how the avg() function is computed
as if it where sum()/count(). Finally, the callback is invoked
at instruction 31.</p>
<p>In summary then, any query with aggregate functions is implemented
by two loops. The first loop scans the input table and computes
aggregate information into buckets and the second loop scans through
all the buckets to compute the final result.</p>
<p>The realization that an aggregate query is really two consequtive
loops makes it much easier to understand the difference between
a WHERE clause and a HAVING clause in SQL query statement. The
WHERE clause is a restriction on the first loop and the HAVING
clause is a restriction on the second loop. You can see this
by adding both a WHERE and a HAVING clause to our example query:</p>
<blockquote><pre>
SELECT three, min(three+four)+avg(four)
FROM examp2
WHERE three>four
GROUP BY three
HAVING avg(four)<10;
</pre></blockquote>
}
Code {
addr opcode p1 p2 p3
---- ------------ ----- ----- ----------------------------------------
0 ColumnCount 2 0
1 ColumnName 0 0 three
2 ColumnName 1 0 min(three+four)+avg(four)
3 AggReset 0 4
4 Open 0 0 examp2
5 Next 0 26
6 Field 0 0
7 Field 0 1
8 Le 0 5
9 Field 0 0
10 MakeKey 1 0
11 AggFocus 0 14
12 Field 0 0
13 AggSet 0 0
14 Field 0 0
15 Field 0 1
16 Add 0 0
17 AggGet 0 1
18 Min 0 0
19 AggSet 0 1
20 AggIncr 1 2
21 Field 0 1
22 AggGet 0 3
23 Add 0 0
24 AggSet 0 3
25 Goto 0 5
26 Close 0 0
27 AggNext 0 41
28 AggGet 0 3
29 AggGet 0 2
30 Divide 0 0
31 Integer 10 0
32 Ge 0 27
33 AggGet 0 0
34 AggGet 0 1
35 AggGet 0 3
36 AggGet 0 2
37 Divide 0 0
38 Add 0 0
39 Callback 2 0
40 Goto 0 27
41 Noop 0 0
}
puts {
<p>The code generated in this last example is the same as the
previous except for the addition of two conditional jumps used
to implement the extra WHERE and HAVING clauses. The WHERE
clause is implemented by instructions 6 through 8 in the query
loop. The HAVING clause is implemented by instruction 28 through
32 in the output loop.</p>
<h2>Using SELECT Statements As Terms In An Expression</h2>
<p>The very name "Structured Query Language" tells us that SQL should
support nested queries. And, in fact, two different kinds of nesting
are supported. Any SELECT statement that returns a single-row, single-column
result can be used as a term in an expression of another SELECT statement.
And, a SELECT statement that returns a single-column, multi-row result
can be used as the right-hand operand of the IN and NOT IN operators.
We will begin this section with an example of the first kind of nesting,
where a single-row, single-column SELECT is used as a term in an expression
of another SELECT. Here is our example:</p>
<blockquote><pre>
SELECT * FROM examp
WHERE two!=(SELECT three FROM examp2
WHERE four=5);
</pre></blockquote>
<p>The way SQLite deals with this is to first run the inner SELECT
(the one against examp2) and store its result in a private memory
cell. SQLite then substitutes the value of this private memory
cell for the inner SELECT when it evaluations the outer SELECT.
The code looks like this:</p>
}
Code {
addr opcode p1 p2 p3
---- ------------ ----- ----- ----------------------------------------
0 Null 0 0
1 MemStore 0 0
2 Open 0 0 examp2
3 Next 0 11
4 Field 0 1
5 Integer 5 0
6 Ne 0 3
7 Field 0 0
8 MemStore 0 0
9 Goto 0 11
10 Goto 0 3
11 Close 0 0
12 ColumnCount 2 0
13 ColumnName 0 0 one
14 ColumnName 1 0 two
15 Open 0 0 examp
16 Next 0 24
17 Field 0 1
18 MemLoad 0 0
19 Eq 0 16
20 Field 0 0
21 Field 0 1
22 Callback 2 0
23 Goto 0 16
24 Close 0 0
}
puts {
<p>The private memory cell is initialized to NULL by the first
two instructions. Instructions 2 through 11 implement the inner
SELECT statement against the examp2 table. Notice that instead of
sending the result to a callback or storing the result on a sorter,
the result of the query is pushed into the memory cell by instruction
8 and the loop is abandoned by the jump at instruction 9.
The jump at instruction at 10 is vestigial and
never executes.</p>
<p>The outer SELECT is implemented by instructions 12 through 24.
In particular, the WHERE clause that contains the nested select
is implemented by instructions 17 through 19. You can see that
the result of the inner select is loaded onto the stack by instruction
18 and used by the conditional jump at 19.</p>
<p>When the result of a sub-select is a scalar, a single private memory
cell can be used, as shown in the previous
example. But when the result of a sub-select is a vector, such
as when the sub-select is the right-hand operand of IN or NOT IN,
a different approach is needed. In this case,
the result of the sub-select is
stored in a temporary GDBM table and the contents of that table
are tested using the Found or NotFound operators. Consider this
example:</p>
<blockquote><pre>
SELECT * FROM examp
WHERE two IN (SELECT three FROM examp2);
</pre></blockquote>
<p>The code generated to implement this last query is as follows:</p>
}
Code {
addr opcode p1 p2 p3
---- ------------ ----- ----- ----------------------------------------
0 Open 0 1
1 Open 1 0 examp2
2 Next 1 7
3 Field 1 0
4 String 0 0
5 Put 0 0
6 Goto 0 2
7 Close 1 0
8 ColumnCount 2 0
9 ColumnName 0 0 one
10 ColumnName 1 0 two
11 Open 1 0 examp
12 Next 1 19
13 Field 1 1
14 NotFound 0 12
15 Field 1 0
16 Field 1 1
17 Callback 2 0
18 Goto 0 12
19 Close 1 0
}
puts {
<p>The temporary table in which the results of the inner SELECT are
stored is created by instruction 0. Notice that the P3 field of
this Open instruction is empty. An empty P3 field on an Open
instruction tells the VDBE to create a temporary table. This temporary
table will be automatically deleted from the disk when the
VDBE halts.</p>
<p>The inner SELECT statement is implemented by instructions 1 through 7.
All this code does is make an entry in the temporary table for each
row of the examp2 table. The key for each temporary table entry
is the "three" column of examp2 and the data
entries is an empty string since it is never used.</p>
<p>The outer SELECT is implemented by instructions 8 through 19. In
particular, the WHERE clause containing the IN operator is implemented
by two instructions at 13 and 14. Instruction 13 pushes the value of
the "two" column for the current row onto the stack and instruction 14
tests to see if top of the stack matches any key in the temporary table.
All the rest of the code is the same as what has been shown before.</p>
<h2>Compound SELECT Statements</h2>
<p>SQLite also allows two or more SELECT statements to be joined as
peers using operators UNION, UNION ALL, INTERSECT, and EXCEPT. These
compound select statements are implemented using temporary tables.
The implementation is slightly different for each operator, but the
basic ideas are the same. For an example we will use the EXCEPT
operator.</p>
<blockquote><pre>
SELECT two FROM examp
EXCEPT
SELECT four FROM examp2;
</pre></blockquote>
<p>The result of this last example should be every unique value
of the two column in the examp table except any value that is
in the four column of examp2 is removed. The code to implement
this query is as follows:</p>
}
Code {
addr opcode p1 p2 p3
---- ------------ ----- ----- ----------------------------------------
0 Open 0 1
1 KeyAsData 0 1
2 Open 1 0 examp
3 Next 1 9
4 Field 1 1
5 MakeRecord 1 0
6 String 0 0
7 Put 0 0
8 Goto 0 3
9 Close 1 0
10 Open 1 0 examp2
11 Next 1 16
12 Field 1 1
13 MakeRecord 1 0
14 Delete 0 0
15 Goto 0 11
16 Close 1 0
17 ColumnCount 1 0
18 ColumnName 0 0 four
19 Next 0 23
20 Field 0 0
21 Callback 1 0
22 Goto 0 19
23 Close 0 0
}
puts {
<p>The temporary table in which the result is built is created by
instruction 0. Three loops then follow. The loop at instructions
3 through 8 implements the first SELECT statement. The second
SELECT statement is implemented by the loop at instructions 11 through
15. Finally, a loop at instructions 19 through 22 reads the temporary
table and invokes the callback once for each row in the result.</p>
<p>Instruction 1 is of particular importance in this example. Normally,
the Field opcode extracts the value of a column from a larger
record in the data of a GDBM file entry. Instructions 1 sets a flag on
the temporary table so that Field will instead treat the key of the
GDBM file entry as if it were data and extract column information from
the key.</p>
<p>Here is what is going to happen: The first SELECT statement
will construct rows of the result and save each row as the key of
an entry in the temporary table. The data for each entry in the
temporary table is a never used so we fill it in with an empty string.
The second SELECT statement also constructs rows, but the rows
constructed by the second SELECT are removed from the temporary table.
That is why we want the rows to be stored in the key of the GDBM file
instead of in the data -- so they can be easily located and deleted.</p>
<p>Let's look more closely at what is happening here. The first
SELECT is implemented by the loop at instructions 3 through 8.
Instruction 4 extracts the value of the "two" column from "examp"
and instruction 5 converts this into a row. Instruction 6 pushes
an empty string onto the stack. Finally, instruction 7 writes the
row into the temporary table. But remember, the Put opcode uses
the top of the stack as the GDBM data and the next on stack as the
GDBM key. For an INSERT statement, the row generated by the
MakeRecord opcode is the GDBM data and the GDBM key is an integer
created by the New opcode. But here the roles are reversed and
the row created by MakeRecord is the GDBM key and the GDBM data is
just an empty string.</p>
<p>The second SELECT is implemented by instructions 11 through 15.
A new result row is created from the "four" column of table "examp2"
by instructions 12 and 13. But instead of using Put to write this
new row into the temporary table, we instead call Delete to remove
it from the temporary table if it exists.</p>
<p>The result of the compound select is sent to the callback routine
by the loop at instructions 19 through 22. There is nothing new
or remarkable about this loop, except for the fact that the Field
instruction at 20 will be extracting a column out of the GDBM key
rather than the GDBM data.</p>
<h2>Summary</h2>
<p>This article has reviewed all of the major techniques used by
SQLite's VDBE to implement SQL statements. What has not been shown
is that most of these techniques can be used in combination to
generate code for an appropriately complex query statement. For
example, we have shown how sorting is accomplished on a simple query
and we have shown how to implement a compound query. But we did
not give an example of sorting in a compound query. This is because
sorting a compound query does not introduce any new concepts: it
merely combines two previous ideas (sorting and compounding)
in the same VDBE program.</p>
<p>For additional information on how the SQLite library
functions, the reader is directed to look at the SQLite source
code directly. If you understand the material in this article,
you should not have much difficulty in following the sources.
Serious students of the internals of SQLite will probably
also what to make a careful study of the VDBE opcodes
as documented <a href="opcode.html">here</a>. Most of the
opcode documentation is extracted from comments in the source
code using a script so you can also get information about the
various opcodes directly from the <b>vdbe.c</b> source file.
If you have successfully read this far, you should have little
difficulty understanding the rest.</p>
<p>If you find errors in either the documentation or the code,
feel free to fix them and/or contact the author at
<a href="drh@hwaci.com">drh@hwaci.com</a>. Your bug fixes or
suggestions are always welcomed.</p>
}
puts {
<p><hr /></p>
<p><a href="index.html"><img src="/goback.jpg" border=0 />
Back to the SQLite Home Page</a>
</p>
</body></html>}
|