diff --git a/manifest b/manifest index 39e76a3a4d..045e94edac 100644 --- a/manifest +++ b/manifest @@ -1,5 +1,5 @@ -C better\scolumn\snames\sin\sthe\sshell\s(CVS\s111) -D 2000-07-29T13:20:21 +C documentation\supdates\s(CVS\s112) +D 2000-07-30T20:04:43 F COPYRIGHT 74a8a6531a42e124df07ab5599aad63870fa0bd4 F Makefile.in 9e6dcd232e594fb599a5e9ba8bcf45e6c6e2fe72 F README 51f6a4e7408b34afa5bc1c0485f61b6a4efb6958 @@ -22,7 +22,7 @@ F src/tclsqlite.c 9f358618ae803bedf4fb96da5154fd45023bc1f7 F src/tokenize.c 77ff8164a8751994bc9926ce282847f653ac0c16 F src/update.c 51b9ef7434b15e31096155da920302e9db0d27fc F src/util.c fcd7ac9d2be8353f746e52f665e6c4f5d6b3b805 -F src/vdbe.c b9ce1439931a56bdbec560d41d32f623b5d4b1c7 +F src/vdbe.c 4308e226d5b33a72dfe2c88a44eb0a63381fe24b F src/vdbe.h 6c5653241633c583549c2d8097394ab52550eb63 F src/where.c 420f666a38b405cd58bd7af832ed99f1dbc7d336 F test/all.test 0950c135cab7e60c07bd745ccfad1476211e5bd7 @@ -63,10 +63,10 @@ F www/changes.tcl 4491a4c835a87945ec4b493d8fed8e31e7917db5 F www/fileformat.tcl f3a70650e942262f8285d53097d48f0b3aa59862 F www/index.tcl 58c9a33ceba12f5efee446c6b10b4f6523a214e1 F www/lang.tcl 1645e9107d75709be4c6099b643db235bbe0a151 -F www/opcode.tcl 401bdc639509c2f17d3bb97cbbdfdc22a61faa07 +F www/opcode.tcl cb3a1abf8b7b9be9f3a228d097d6bf8b742c2b6f F www/sqlite.tcl 69781eaffb02e17aa4af28b76a2bedb19baa8e9f -F www/vdbe.tcl 3330c700ef9c212a169f568a595361e4cce749ed -P 3bf434d93a54a24f4882d0d9375f82ceee0b7602 -R 62d5ed1a73773f8306d7f2d4bcc6586d +F www/vdbe.tcl bcbfc33bcdd0ebad95eab31286adb9e1bc289520 +P 57022a9d504e553d862f363b164c42ba53d8b489 +R f7bf520ee9c56f3008ca4098d4e4be60 U drh -Z 12cb62dbb78ef20fcb0d4c8ab57670db +Z d5fb118e39f2c1e4c12a967c5d47998a diff --git a/manifest.uuid b/manifest.uuid index e4e793efbd..aa5608d8f2 100644 --- a/manifest.uuid +++ b/manifest.uuid @@ -1 +1 @@ -57022a9d504e553d862f363b164c42ba53d8b489 \ No newline at end of file +c686c6076abadcb715fe74436fa8bab48d013b26 \ No newline at end of file diff --git a/src/vdbe.c b/src/vdbe.c index 0d916e0c48..34240b7d3d 100644 --- a/src/vdbe.c +++ b/src/vdbe.c @@ -41,7 +41,7 @@ ** 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 $ +** $Id: vdbe.c,v 1.37 2000/07/30 20:04:43 drh Exp $ */ #include "sqliteInt.h" #include @@ -1970,7 +1970,7 @@ int sqliteVdbeExec( /* 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 + ** 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. */ diff --git a/www/opcode.tcl b/www/opcode.tcl index c4822b3675..abc96d762b 100644 --- a/www/opcode.tcl +++ b/www/opcode.tcl @@ -1,7 +1,7 @@ # # 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 $} +set rcsid {$Id: opcode.tcl,v 1.4 2000/07/30 20:04:43 drh Exp $} puts { @@ -55,8 +55,8 @@ by the SQLite library. This document describes the operation of that virtual machine.

This document is intended as a reference, not a tutorial. -A separate Virtual Machine Tutorial is currently -in preparation. If you are looking for a narrative description +A separate Virtual Machine Tutorial 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 diff --git a/www/vdbe.tcl b/www/vdbe.tcl index 14e259911c..cf47294f2b 100644 --- a/www/vdbe.tcl +++ b/www/vdbe.tcl @@ -1,7 +1,7 @@ # # 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 $} +set rcsid {$Id: vdbe.tcl,v 1.5 2000/07/30 20:04:43 drh Exp $} puts { @@ -14,11 +14,12 @@ The Virtual Database Engine of SQLite puts "

(This page was last modified on [lrange $rcsid 3 4] GMT)

" -puts { -
This document is -currently under development. It is incomplete and contains -errors. Use it accordingly.
-} + +# puts { +#
This document is +# currently under development. It is incomplete and contains +# errors. Use it accordingly.
+# } puts {

If you want to know how the SQLite library works internally, @@ -96,6 +97,9 @@ INSERT INTO examp VALUES('Hello, World!',99);

We can see the VDBE program that SQLite uses to implement this INSERT using the sqlite command-line utility. First start up sqlite on a new, empty database, then create the table. +Next change the output format of sqlite 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 sqlite to print the VDBE program rather than @@ -153,7 +157,7 @@ another cursor open for writing that same file.

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 +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.

@@ -197,7 +201,7 @@ stack {A data record holding "Hello, World!" and 99} \ {A random integer key} puts {

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 +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.

@@ -301,7 +305,9 @@ int Callback(void *pUserData, int nColumn, char *azData[], char *azColumnName[])

The SQLite library supplies the VDBE with a pointer to the callback function -itself, and the pUserData pointer. The job of the VDBE is to +and the pUserData pointer. (Both the callback and the user data were +originally passed in as argument to the sqlite_exec() API function.) +The job of the VDBE is to come up with values for nColumn, azData[], and azColumnName[]. nColumn is the number of columns in the results, of course. @@ -755,18 +761,859 @@ table. This text is fed back into the SQLite parser and used to reconstruct the internal data structures describing the index or table.

-

Using Indexes To Speed Searches

-TBD +

Using Indexes To Speed Searching

+ +

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.

+ +

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.

+ +

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:

+ +
+SELECT * FROM examp WHERE two==50;
+
+ +

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:

+ +
+SELECT * FROM examp WHERE two<50;
+
+ +

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.

+ +

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:

+ +
+CREATE INDEX examp_idx1 ON examp(two);
+
+ +

The VDBE code generated by the above statement looks like the +following:

+} + +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 { +

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.

+ +

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.

+ +

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. +

+ +

Now let's look at how this index will be used. Consider the +following query:

+ +
+SELECT * FROM examp WHERE two==50;
+
+ +

SQLite generates the following VDBE code to handle this query:

+} + +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 { +

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.

+ +

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.

+ +

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.

+ +

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:

+ +
+INSERT INTO examp VALUES('Hello, World!',99);
+
+ +

And the generated code looks like this:

+} + +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 { +

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.

+

Joins

-TBD + +

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.

+ +

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:

+ +

+

    +
  1. Initialize the azColumnName[] array for the callback.
    2. +
  2. Open two cursors, one to each of the two tables being queried.
    3. +
  3. For each record in the first table, do: +
      +
    1. For each record in the second table do: +
        +
      1. If the WHERE clause evaluates to FALSE, then skip the steps that + follow and continue to the next record.
        2. +
      2. Compute all columns for the current row of the result.
        3. +
      3. Invoke the callback function for the current row of the result.
        4. +
      2. +
    +
  4. Close both cursors.
    5. +
+

+ +

This template will work, but it is likely to be slow since we +are now dealing with an O(N2) 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:

+ +

+

    +
  1. Initialize the azColumnName[] array for the callback.
    2. +
  2. Open two cursors, one to each of the two tables being queried.
    3. +
  3. For each record in the first table, do: +
      +
    1. 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.
      2. +
    2. For each record in the second table do: +
        +
      1. If the WHERE clause evaluates to FALSE, then skip the steps that + follow and continue to the next record.
        2. +
      2. Compute all columns for the current row of the result.
        3. +
      3. Invoke the callback function for the current row of the result.
        4. +
      3. +
    +
  4. Close both cursors.
    5. +
+

+ +

Additional speed-up can occur if an index can be used to speed +the search of either or the two loops.

+ +

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.

+ +

You can see how SQLite constructs nested loops in the following +example:

+ +
+CREATE TABLE examp2(three int, four int);
+SELECT * FROM examp, examp2 WHERE two<50 AND four==two;
+
+} + +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 { +

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.

+ +

SQLite does not impose any arbitrary limits on the tables in +a join. It also allows a table to be joined with itself.

+

The ORDER BY clause

-TBD + +

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.

+ +

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.

+ +

We can see the process in action in the following query:

+ +
+SELECT * FROM examp ORDER BY one DESC, two;
+
+} + +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 { +

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.

+ +

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.

+ +

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 "+".

+ +

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.

+

Aggregate Functions And The GROUP BY and HAVING Clauses

-TBD + +

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.

+ +

An example will help to clarify this concept. Consider the +following query:

+ +
+SELECT three, min(three+four)+avg(four) 
+FROM examp2
+GROUP BY three;
+
+} + +puts { +

The VDBE code generated for this query is as follows:

+} + +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 { +

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.

+ +
+ + + + + +
Memory SlotIntended Use Of This Memory Slot
0The "three" column -- the key to the bucket
1The minimum "three+four" value
2The number of records with the same key. This value + divides the value in slot 3 to compute "avg(four)".
3The sum of all "four" values. This is used to compute + "avg(four)".
+ +

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.

+ +

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.

+ +

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.

+ +

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:

+ + +
+SELECT three, min(three+four)+avg(four) 
+FROM examp2
+WHERE three>four
+GROUP BY three
+HAVING avg(four)<10;
+
+} + +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 { +

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.

+

Using SELECT Statements As Terms In An Expression

-TBD + +

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:

+ +
+SELECT * FROM examp
+WHERE two!=(SELECT three FROM examp2
+            WHERE four=5);
+
+ +

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:

+} + +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 { +

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.

+ +

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.

+ +

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:

+ +
+SELECT * FROM examp
+WHERE two IN (SELECT three FROM examp2);
+
+ +

The code generated to implement this last query is as follows:

+} + +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 { +

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.

+ +

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.

+ +

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.

+

Compound SELECT Statements

-TBD + +

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.

+ +
+SELECT two FROM examp
+EXCEPT
+SELECT four FROM examp2;
+
+ +

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:

+} + +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 { +

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.

+ +

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.

+ +

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.

+ +

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.

+ +

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.

+ +

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.

+ +

Summary

+ +

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.

+ +

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 here. 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 vdbe.c source file. +If you have successfully read this far, you should have little +difficulty understanding the rest.

+ +

If you find errors in either the documentation or the code, +feel free to fix them and/or contact the author at +drh@hwaci.com. Your bug fixes or +suggestions are always welcomed.

} puts {