5a2a287752
* *
* partial sync with the 3.4 fork codebase. These are the things
synces for the most part:
- copyright headers
- grammar/typos in comments and some readmes
- comment/whitespace/decorations
- variable scoping in C files
- DO CASE/SWITCH and some other alternate syntax usage
- minimal amount of human readable text in strings
- minor code updates
- HB_TRACE() void * casts for pointers and few other changes to
avoid C compiler warnings
- various other, minor code cleanups
- only Harbour/C code/headers were touched in src, utils, contrib,
include. No 3rd party code, no make files, and with just a few
exceptions, no 'tests' code was touched.
- certain components were not touched were 3.4 diverged too much
already, like f.e. hbmk2, hbssl, hbcurl, hbexpat
- the goal was that no actual program logic should be altered by
these changes. Except some possible minor exceptions, any such
change is probably a bug in this patch.
It's a massive patch, if you find anything broken after it, please
open an Issue with the details. Build test was done on macOS.
The goal is make it easier to see what actual code/logic was changed
in 3.4 compared to 3.2 and to make patches easier to apply in both
ways.
126 lines
5.2 KiB
Plaintext
126 lines
5.2 KiB
Plaintext
Simple SQL Interface for Harbour
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1. Introduction
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Simple SQL interface implements accessing SQL query result via RDD
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interface. It is not intended to be replacement for "transparent" move of
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DBFCDX application to SQL world.
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I want to discuss this in more detail. Many current RDDs for SQL servers
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(ex. SQLRDD from xHarbour.com) tries to make a feeling you are working with
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DBF file, but not with SQL database. SQL server does not support many
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features, ex. RecNo(), deleted flag, file locks, record locks. These RDDs
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are emulating these features to make feeling of DBF. Deleted() function is
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emulated by creating additional table columns to store delete flag. Some
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"hidden system" tables are used to register locking operations and emulate
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record and file locks in DBF style. The idea of SQL query is also lost. If
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you do a simple loop
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dbUseArea( , "select * from my_table" )
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DO WHILE ! Eof()
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somefunc( FIELD->some_sql_field )
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dbSkip()
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ENDDO
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RDD usualy will read SQL rows in portions, let's say 100 records per query.
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So, hidden queries are generated. If you are using indexes these queries
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are really complicated. Let's have index on FIELD1 + Str( FIELD2 ). A seek
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to value cValue1 + Str( nValue2 ) will generate a query like:
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SELECT * FROM my_table
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WHERE (FIELD1 == cValue1 and FIELD2 >= nValue2) or FIELD1 > cValue1
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ORDER BY FIELD1, FIELD2, _RECNO
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LIMIT 100
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After evaluation of first 100 cached records, next query will be generated:
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SELECT * FROM my_table
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WHERE (FIELD1 == cLastField1 and FIELD2 == nLastValue2 and _RECNO > nLastRecno) or
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(FIELD1 == cLastField1 and FIELD2 > nLastValue2) or
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FIELD1 > cLastValue1
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ORDER BY FIELD1, FIELD2, _RECNO
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LIMIT 100
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To optimize these queries the SQL index expresion should be
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"FIELD1,FIELD2,_RECNO", but not "FIELD1,FIELD2" as written in INDEX ON
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command.
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"Simple SQL interface" is too long to repeat every time I want to
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address this library. I'll also use acronym "SSI" to address it.
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The idea of SSI is different. It does not make hidden queries. All
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queries should be made explicitly by programmer. SSI gives access to query
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result via RDD interface, it does not tries to emulate DBF and be
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"plug-and-play" solution for DBF to SQL migration. If you do
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dbUseArea( , "select * from my_table")
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all query (it could contain millions of records!) will be cached.
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The features of SSI approach are:
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- It's possible to access SQL database of other applications. Other
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applications usualy does not follow agreement of "plug-and-play" SQL drivers
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about additional DELETED column, _RECNO in the end of index expression, etc.
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Access of SQL database of other applications is sometimes not possible.
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- It's query oriented. That means a simple DO WHILE ! Eof() loop will iterate
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each records once and only once. This is not true for "plug-and-play" SQL
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drivers, if indexing is used. Just like in the case of loop over DBF file.
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It is not guaranteed that all records are included! Yes! If key value of the
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first record in index is changed to be the last record in index during the
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phase of record processing, DO WHILE ! Eof() loop will iterate only this
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single records even if the database contains millions of records. Your sould
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do FLock() on DBF to guarantee the records are not changed. Do you use FLock()
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before readonly DO WHILE ! Eof() loops? :)
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2. Architecture
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+-------------+
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| SQLMIX RDD |
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+-------------+
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| ^
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V |
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+-------------+ +---------+
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| |--->| |
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| SQLBASE RDD | | SDD |
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+-------------+ +---------+
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SQLBASE RDD implements basic functionality for accessing SQL query result
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via RDD interface. This RDD could be used, if indexing of query result is not
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necessary or all indexing is done by SQL server (by using ORDER BY clause).
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SQLMIX RDD implements indexing of query result. This indexing is not
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related to SQL server ORDER BY clause. SQLMIX do indexing of the query on the
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client side.
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SDD is acronym for Sql Database Driver. RDD is used to implement access
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of different database formats like DBF, SDF, etc. SDD is used to implement
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access of different SQL databases. Every SQL server (MySQL, PostgreSQL, etc.)
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has a corresponding SDD. SDD driver implements a specific part of data
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exchange interface between SQLBASE and SQL server.
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3. Modifying database
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SSI presents a query result via RDD interface and generates no hidden
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SQL queries. So, how database can be changed? Does dbAppend() and FieldPut()
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works, or is it readonly SQL interface?
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dbAppend(), FieldPut() and other similiar functions work on cached query
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result, i.e. query can be appended by new rows and field values can be
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changed, but SQL database is not changed. dbCreate() function can also be
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used to create an "empty query result" but no table is created on SQL server.
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So, SSI can also be used as implementation of "array RDD".
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The programmer must call SQL command explicitly to modify SQL tables.
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SSI provides a method to detect which cached rows was changed or appended.
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